You could try CalcTape. Free version uses about 30mb, does percentages and appears to do variables as well. It saves everything on a tape and you can edit the numbers and it will recalculate. You can purchase the pro version for $6.95us for more features.

I have seen a lot of folks here praise Opalcalc as @flamerz said. I generally am satisfied with the Windows or Android calculator so you have to make your own value judgements.  ;D

I know I had Caffeine on a short list too. There was another one I considered, but I don't recall the name - but it wasn't on the list at Sphere's link.

The easy way is click on the note in the upper left that says "You have $0.00 in DonationCredits":



Go through Paypal to get some credits. Then you can donate by clicking the coin under the person's name:



The value is up to you! How much do you appreciate what was done for you? How much effort will this save you? If you bought this from a store what would it cost you? Think about those guidelines, just remember it is a token of your gratitude. I think a lot of folks here get real enjoyment for writing a program to fulfill a need, but maybe a cup or two of joe would make their day nicer!