This year for my taxes I used a little DSL I hacked up in TXR Lisp, where I you can declare each line on a tax form, along with its description and its value (or else how it is calculated from other lines). A bit like cells in a spreadsheet: reactive programming, basically.

There is a function to produces a report of all the lines and their values, grouped by form, sorted by line.

If I change a value, as in (set (line 42) 123.32), then re-run the report, it will indicate all the lines that have changed, showing the old and new value side by side.

I have some nice things in there, like handling groups of conditional lines that vary by tax bracket and such.

All of the data is in a nice text file that is just (load ...)-ed, together with the module that provides the logic, and that file is checked into git.

If the government did these calculations, I'd still want the option to do it myself; I wouldn't want to be forced into dealing with entering numbers into some web crap.