I almost always at least double the tax, which gives a tip of about 18 percent. If I don't remember the tax and it isn't on the receipt I might just pay twenty percent of the check with tax included, and will add something if any member of my party has been obnoxious or unreasonable.
I always figure that if tips are built into the wages they probably don't have great health insurance or much sick time either. It actually feels nice to be nice to someone who has been nice to you, and I will give slack to anyone who is rushing around and very busy.