The math?
Get a tip calculator, like MortY Seinfeld. 
Let's see...10% is easy, halve it and add that to 10 to get 15, double it for 20. 18% Automatic is an insult for those try hard and a gimme for lazy servers. Not that 18 is a bad tip, but why should Cindy Slow and Rude get the same consideration, tip-wise, as Oliver On the Ball?
As i often heard after tipping "you've done this kind of work, haven't you?" Why yes, yes I have, I've waited on collosal entitled snot-noses too. Aren't us nice people refreshing?