15% is usually what I pay, normal or above-average service.
I'd like to pay more, but I'm a poor college student. 15% of a $50 bill already makes it nearly $60, which is too rich for my blood.
20% if extraordinarily good service, friendly, and I get a liking to them.
0% if below average or bad service.
I tend to only give out those 3 percentages.