For average service, I tip 12 percent on the bill total. To offset the tax added. 
If someone is shit, I give 'em a good three or four percent, plus a written note on the receipt to get fucked. Usually accompanied with a frowning face. 
Like another says, they're taking my order, bringing me a plate and checking in on me at some point to see if I'm OK. Maybe top up my root beer. All with a smile. 
Big deal. 
I'll be equally pleasant and toss ya 12 percent. Now move along. 
Fortress has shit to do.