20% is easy to calculate and a decent tip, but I sometimes leave more if the service is really good.
Sometimes I'll give more. I gave a rehearsal dinner for my son's wedding and the tab came to $3,300. I signed off for $5,000, as that was my budget. They ended up with just over a 55% tip that night. But then again, how often does your son get married?
I've worked in restaurants. If you give a lousy tip, don't expect to ever go back there. If they recognize you as a lousy tipper, you are going to regret it. I've seen some horrible things done to food in restaurants that is going to difficult customers or lousy tippers. Not saying it's right but I would not want to be on the receiving end if you know what I mean.