Best way to do it would be to build in a checksum. Every digit in the final amount after tip should add up to a number only known to you. We'll use 25 as an example. If you get a $80 bill, with a 20% tip, you'd be at $96. 9+6 = 15, so you can round up $0.19 so that your total is $96.19. 9+6+1+9 = 25, your secret checksum.

What if you don't want to tip an extra few cents every time? well you could also round down $0.08 to 95.92 in our example. 9+5+9+2 = 25.

Problem solved (and needlessly over-complicated)