I usually tip around 20% at restaurants unless the service is bad just because that is what is considered socially acceptable, but I never really understood why the tip is based on the total bill. To me it seems like it would make more sense to tip by the number of plates. If I go out with my girlfriend and we both order $20 steaks, the waitress isn't doing any more work than she would be if we both ordered a $7 cheeseburger, so why should the tip be more than twice as much?