I think in many situations tip calculators focus on the wrong thing. They are answering the question "How much do I pay given the tip percent?"
Often the actual question I want answered is "What tip will I be giving if I pay various whole dollar amounts?"
For example, let's say I order a pizza for delivery, and the mandatory charges come to $20.60, and let's also say I consider 15% a reasonable tip.
A normal tip calculator will tell me that I should give the pizza guy $23.69.
Screw that, I'm not futzing around with trying to find $0.69 in coins, and I'm certainly not going to give him $24 or $25 and ask for $0.31 or $1.31 in change.
Nope. I'm going to pay an integral number of dollars and say "keep the change".
What I want a tip calculator to do is give me a table:
Pay Effective Tip
$21 1.9%
$22 6.8%
$23 11.7%
$24 16.5%
$25 21.4%
I can then look at that, and see that my best choices are $23, $24, or $25. $23 comes out far enough below 15% that I'll cross that off. $24 is a little over 15%, so is perfect, but if I don't have 4 $1 bills in my wallet, $25 would not be extravagant. Result: $24 or $25, depending on exactly what bills are in my wallet.
I made an Apple Dashboard widget to do just that, and it worked out quite well. If I had ever gotten around to learning mobile development, that was going to be my first app for learning.