20% for excellent service.
It goes down from there. Yes zero tip is acceptable if the service sucked. If I ordered medium rare steak and I get well done steak. I normally wonβt deduct that from the tip since that is a hard one for the server to see. But if itβs something they could have seen and didnβt fix, yeah Iβm probably reducing the tip.
The tip is for service above and beyond, not a required part of the bill.