Mom was a waitress for over 40 years at Dennys alone, and tipped 15% unless the service was really bad. Then she'd tip two pennies. The message was "The service wasn't worth two cents but I feel too guilty not tipping at all." She started waiting tables when a dime was a good tip.Tight azz.
If you got someone waiting on you at a table and the service ain’t deplorable, tip 20%.
If the service is deplorable, they get nothing. Find a different gig.