I usually give 20%. Recently went to a restaurant that added 18% to the check. It was just my wife and I. I didn't like them adding it without notices before hand. But I figured I saved 2%.
Cracks me up when they inform me that they have included the gratuity in the bill.............. and then tell me that I can add more.
Had they left it to me they would have gotten more, if you add it on yourself then I assume you know what your service was worth..............