How Do I Calculate a Restaurant Tipping Bill?

Sen. Rand Paul says he can't stand the idea of tipping at a self-service restaurant. He thinks 20 per centi is the standard for decent service, and that it's just flattery. But when you go to a counter service restaurant,. normally tipping, you leave a tip. Sen. Paul: "I don't know what is, computer, tell me right now. What is?"