i work in a restaruant, i'm a chef, so i get to listen to people bitch about bad tips all day. here's how it is.
if my server is good i always leave AT least 20% the reason i like to tip high i because it's these's people livelyhoods, also if you frequent there, the next time you come back they will remember you and you will get twice the service.
if the service is shitty fuck em.
5 dollars on 31 is a good tip, it's right around what thy would hope to get on the meal, 10 - 15%. surprisingly enough, about 50% of people don't tip more than 5%.
so bottom line, at least 10% even if the service is sub standard, anything more than that, judge it for yourself. how much was their attentiveness worth to you?