Start off by installing it.  On Fedora 9 you can do this by selecting System → Administration → Add/Remove Software from the menu and searching for “expendable”.  If you are using Fedora 8, select Applications → Add/Remove Software.

After installing it, you can run the program by selecting Applications → Office → Expendable from the main menu.  You should see something like this (click for a close-up):

A straightforward mortgage

This example data shows how the program can be used straight away.  Let’s say we have a mortgage with Nationwide, with an outstanding balance of £80,500 and 12 years 3 months left to run. (These figures will be on your most recent mortgage statement.)

Click on ‘My Mortgage’ in the account name field on the left hand side, about half-way down the screen, and replace that text with “Nationwide Mortgage”.  Let’s say the interest rate is 6.5% APR: use the up-arrow button next to the interest rate field to set that.  You’ll notice the graph changing as you do this.  Now adjust the loan amount to £80,500 (you’ll have to type in “80500″, as the up/down arrows make changes in increments of £1,000).  Now use the arrow buttons on the next line down to adjust the term to 12 years and 3 months.  Your window should now look like this:

As you can see, the graph has been updated to reflect the details we just entered.  The horizontal axis shows time moving forward, with 5-year intervals marked.  The red line shows the outstanding mortgage balance over time, decreasing more quickly when the loan is nearly repaid.  The green line shows the total amount of interest paid.

Below the graph are some figures illustrating this.  The “overview” part shows the total amount of interest paid.  Click on the “Nationwide Mortgage” part to see more details about the mortgage:

Using an offset facility

So much for the mortgage; now let’s try something interesting.  We can find out the effect on the mortgage if we are able to overpay each month by, say, £20.  To do this, click on ‘Monthly overpayment’ in the box on the left hand side.  Below the box, adjust the ‘amount each month’ field to 20.  Now there is a blue line on the graph, and this illustrates the amount in the offset facility of the mortgage.

Paying an extra £20 each month into a mortgage that has an offset facility increases the amount in the offset by that much.  It is effectively a savings account, but rather than paying interest on the amount in the offset account, the amount of interest charged on the mortgage is reduced as though the loan amount were smaller.

As you can see, the loan term has now shortened by 5 months, to 11 years and 10 months.  This is a saving in mortgage interest of over £1,500.  When the blue line (the offset account) hits the red line (the outstanding loan amount), they both drop to zero — at that point, the loan has been repaid.

You can save this model by selecting File → Save As… from the menu.

Savings account with fixed term deposit

Now let’s try an example using a savings account as the main focus.  This time, we have £10,000 in a savings account paying 6.5% AER, but we want to put half of it into a high-interest fixed term deposit.  The FTD pays 10% AER but locks away the money for a year.

To set this up, select File → New from the menu, choose Savings Account, and click OK.

Name this account “Savings” and set the rate and balance.  Leave the tax rate at 20% — this means that our savings interest is taxed at the basic rate.  The graph is drawn showing an ever-increasing balance.  By default, the first 30 years are modelled, and for this example we only want to see the first year.  Adjust the ‘Limit’ fields at the top left of the window, below the ‘Run model’ checkbox, to “1 yrs 0 mths”.  Notice that below the graph, the final balance of the savings account is shown in the “Savings” tab: £10,516.97.

Now we want to add a fixed term deposit.  Select Edit → Add Account from the menu and choose Fixed Term Deposit, then click OK.

Name this account “FTD” and set the rate to 10.  Now adjust the amount up to 5000.  The final balance in the savings account after 1 year is now shown as £10,677.67, and by flipping between the “Savings” and “FTD” tabs below the graph we can see that this is due to £281.20 in interest from the savings account and £396.47 from the fixed term deposit.

This example shows a bit more about how the accounts tree works.  The fixed term deposit account takes its initial balance from its “parent” account, the savings account.  When the term is up, it pays back into its parent account.  This sort of thing works for mortgages too.  You can set up quite complicated arrangements if you wish, for example a mortgage which will be paid off by a savings account, which in turn has several linked fixed term deposits.  The example data file shown when you first start the program gives a good starting point for experimentation.

You can also use this example to see the effect of waiting 6 months before setting up the fixed term deposit, as follows.  First, increase the limit to 2 years.  Next, find the “Start: 0 mths late” part and adjust it to “6 mths late”.

Savings account with regular saver

Here’s another example using a savings account.  In this case, we’ll be “drip-feeding” a regular saver account from a savings account.  A regular saver account is similar to a fixed term deposit, but rather than setting it up from one lump sum at the beginning, monthly contributions are made.  Typically these last for a year and the maximum monthly contribution is £250.  The money is locked away for that time.  For our example, the savings account pays 6.5% AER, we have £10,000 in it, and the regular saver pays 10% AER and we want to pay £250 into it each month.

Martin Lewis’s web site has a calculator for modelling just this type of situation.

So, here’s how we’ll set it up: first, select File → New from the menu and choose to create a savings account.  Give it a name, and set the rate to 6.5 and balance to 10000.  Now select Edit → Add Account and add in a Regular Saver.  Give that a name and set the rate to 10, and the monthly amount to 250.  Set the limit to 1 year to see the result: the total interest earned in the first year is £542.44, with £434.92 of that coming from the normal savings account and £107.52 from the regular saver.

Using up savings

Finally, let’s try an example in which we have savings, but are spending them.  The questions we need to know the answers to are: how much can we overspend by each month, and how long can that last before the savings run out?

We’ll use the same data as in the previous example, but we’d better make sure we set the “minimum amount” for the regular saver.  If there is not enough money to contribute the monthly amount (£250), the minimum amount will be contributed.  Let’s say that in this example, there is no flexibility at all: the minimum is £250 as well.

To simulate spending with abandon, add another account, this time of type “spending”.  Now, click on the up-arrow next to “amount each month” until the amount stops changing.

A vertical line is drawn at the point in time where spending any more each month will result in the savings running out: 11 months from now, if the monthly shortfall is £590.

Feedback please!

There are lots of ways of combining different account models, and lots of different types of questions that can be answered by using this modelling program.  If you have a problem getting it to behave the way you want it to, or have an idea for making it better, please let me know by adding a comment below.

There are mortgage lenders that let you manage offset accounts on-line, and even show nice graphs to show the predicted effect of offsetting (i.e. how much less time it will take to pay off the mortgage) but this graph is a lot more configurable than any I’ve seen.

You can use it to say that you’ve a certain amount in the offset account, but you’ll be withdrawing an amount each month; or you expect a fixed term deposit to mature in 6 months; or there’s a big purchase on the horizon that will reduce the size of the offset.

The project is called expendable, and I’m hosting it at expendable.fedorahosted.org.  Click on the picture above to take a closer look.

I found the vnstat project, which is already packaged for Fedora, and this is almost exactly what I needed.  After installing the package, I just needed to configure it:

1. Edit /etc/sysconfig/vnstat to make sure the correct network interface is monitored.
2. Edit /etc/cron.d/vnstat and uncomment the line that enables network interface monitoring (not sure why this comes commented out).
3. Edit /etc/vnstat.conf and check that everything looks right. I changed the locale, but I’m not sure why it doesn’t use the system locale.  Maybe something to do with cron?

It works by gathering data from the proc filesystem and using it to update a simple database file each time it is run from cron.

The only missing piece was the ability to examine network usage during peak hours only.  I’ve sent a patch for this to the maintainer, and put a small git repository here:

git clone http://cyberelk.net/tim/data/vnstat.git

If you’re reading this from the future and that link doesn’t work, hopefully it’s because the patch has been accepted…

I can now see my actual peak usage so far, as well as the estimated total for this month.  Here’s an example of the output (not real data):

 eth0  /  monthly

   month         rx      |      tx      |   total
-------------------------+--------------+--------------------------------------
  Jul '08       1.34 GB  |   103.00 MB  |     1.44 GB   %%%%%%%%%%%%%%%%%%%%::
   (peak)      32.57 MB  |     1.71 MB  |    34.28 MB
-------------------------+--------------+--------------------------------------
 estimated     16.10 GB  |     1.21 GB  |    17.31 GB
   (peak)        384 MB  |       12 MB  |      396 MB

Its purpose is to ask the user for a fax number when it receives a D-Bus signal from the hpfax backend. That’s all — so why does it sit there with a big fat ‘HP’ logo all the time? Unlike Windows, our notification area is not an advertising board.

Not that the logo is even in the notification area. Instead, it gets placed like a window, just anywhere there is space on your desktop. Nice.

That’s all disregarding the fact that none of this is any use at all if your fax queue resides on a networked CUPS server. It can send D-Bus signals if it likes, but they won’t be heard over the network.

Unfortunately it doesn’t seem to cope very well with having multiple ChangeLog entries added on the same date without separate date stamps, and I don’t know how to tell emacs to always add a new date stamp line when using it to add ChangeLog entries (C-x 4 a).

There is also support for proxy authentication. This is when you have submitted a print job to a queue for a network printer, but the remote printer requires authentication. When this happens, the CUPS backend responsible for transporting the job to the network printer stops and waits for authentication details. In system-config-printer 1.0.0, the print applet now spots this, displays an authentication dialog, and passes this to CUPS for it to complete the job.

This works for the ipp backend, but not yet for the smb backend. The samba v3-2-test branch has code to deal with this, but it doesn’t yet work for me (although I have a one line change that fixes it for me). As a result, the “prompt user for authentication” option just has the effect of not providing any authentication details at all at the moment.

Meanwhile, the job is held in the queue:

It resumes processing once the authentication details are supplied. Still to do: use GNOME Keyring to store the details, to prevent having to enter them over and over again when printing several jobs.

Here are some of the improvements:

• The configuration tool window has been made a lot simpler.  Instead of a list of printer names on the left and properties for the selected printer on the right, now there is just a box containing icons representing printers.  Double-clicking on a printer opens a properties dialog.
• The CUPS authentication dialog now selects the appropriate user-name, and allows it to be altered mid-operation.
• When the configuration tool is running, its list of printers is now updated dynamically.
• All jobs queued for a specific printer can be seen by right-clicking on a printer icon and selecting ‘View Print Queue’.  To see jobs queue on several printers, select the desired printers first before right-clicking.  To see all jobs, right-click with no printers selected.
• The job monitoring tool now displays a message when a job has failed.  If the printer has been stopped as a result, this is shown in the message.  A “Diagnose” button starts the trouble-shooter.
• The job monitoring tool now performs proxy authentication.  When a job is submitted but the CUPS backend requires further authentication information from the user, it stops the job.  In this situation, the job monitoring tool will now display an authentication dialog so that the job can proceed.

When 1.0.0 is released it will make its way into Fedora 10.  Depending on what sort of shape it is in, it might be released as an update for Fedora 9.

The printer properties and server settings are now in separate dialog boxes, although it doesn’t all work correctly yet.

I’ve checked this code in to the ‘new-ui’ branch.