Some of the guidelines offered are very good:
10% For Average Buffet, 15% for an Excellent Service

At our local Chinese resturaunt we generally tip $1 per person. This works out to a little more than 10%, but they treat us very well. It dawned on me that one of the reasons we are treated so warmly was that we do tip a decent amount. I am amazed at the numbers of large groups who only leave a $2 tip. The waitress has been kept very busy refilling glasses, clearing plates, etc. for a large group of people and she is given an insulting tip.

The resturaunts that add in my tip for me usually shortchange themselves. My standard is 20%, unless the service is terrible.