I generally tip 20% unless the service was shitty.

One time, in an almost empty burger bar, I had a horrible meal with a friend of mine. It was horrible mostly because of the shitty service we got from our waitress who would walk right past us and ignore us waving at her when we had empty beer glasses. Then it took 20 minutes to get our final check after we asked for it. My friend paid with a credit card and she got a tip of exactly $0.01.