The only two things in life that are for sure are death and taxes

We cannot help you with death but Pentacalc can help with taxes.

Calculating Tax

You may store a tax rate by entering the new tax rate as a percentage and pressing [Tax Rate]. The rate will be stored until a new tax rate is entered. To look at the current tax rate, press [Tax Rate], and the tax rate will be displayed in the entry display. Tax is computed on the current running total.

To calculate tax, press [Tax]. The amount of tax is displayed on the tape, but then you must press either [+] or [-] to have it added to or subtracted from the current running total. You may also calculate tax on only one item in a list. This is illustrated in the first example below. The default tax rate is 6.25%.