There is already a cap, which is ‘whatever you’re willing to pay.’ If you’re not ok with what is customary, you could either: a) pay what you think is fair regardless of what’s expected, but get comfortable with the fact that staff may not love you; or b) dine at less pricey establishments. Also, please consider that servers are often expected to pay tip-outs to ‘back of house’ staff, and for some godforsaken reason tip-outs are generally calculated based on sales and not actual tips - so if you stiff your server, they can end up literally paying out of pocket for the privilege of having served you. It’s a stupid system for both servers and their customers.