just calculate the tip amount based on bills - 20-25%.
I do same. 15% is the minimum. For group - 18%. Today - we went to "Omelette Headquarter" near Boston for breakfast and the breakfast was omfgwtf. Real Egg, Real Bread, Real shit. Waitress was very nice because we're all deaf. Since the food was damn good and I was in good mood, I dropped 25% tip.
Same for yesterday (just 18%) even though the food was shit but the waitress was a very gracious host - all 8 deafies including me. I had a good time.
