IMO restaurant tips (and other service businesses) are 15% by default, 20% if they do well, 10% if they do poorly. If they do especially poorly (like, completely ignoring the table for an hour while chatting with coworkers off to the side), they get $.02. If they do especially well, more than 20% (I've gone as high as 50% once).

This mentality of doing 20% by default for something you already paid is what got us in this situation.

People should be paid a living wage by default.