but I normally start at 20% for breakfast, 15% for lunch and 10% for dinner. At dinner if they do their job (I used to be a waiter) I up it to 15%, if they do a great job I give 25% and if they are 1 in a million then more. I don't ever stiff them. But if the water glass isn't alway 3/4 full they are not doing the job IMHO