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.
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%.