I usualy do 15% IF the service is excellent. Less if the service is average and nothing if the service is poor.
The worst tip I ever left was at a BBQ restaurant. We, about 10 of us, were having a business meeting and we broke up for lunch. We all went to the same restaurant, but broke up into table sized groups so we wouldn't have a long wait. We were there about 5 or so minutes when a waiter came sauntering by. We tried to get his attention, three times but he just kept walking by. The fourth time my friend tapped his arm as he went by. He turned to look at us and my friend asked for some iced tea and some water for the other two of us. The waiter replied, "When I get time, I'll come back." and abruptly turned and left. It went downhill from there. I finally got up, folded my napkin over my arm and went to the waiter's station, got a pitchure of water and of iced tea and three glasses and went back to our table. (I'm dressed in a log sleeved white shirt, business tie, and suit pants.) I poured the iced tea and water and sat down. The waiter came over and asked who left the pitchures of iced tea and water on our table? I told him I did and he got mad and basically said, "you can't do that". (I had to get water and refill our glases once more.) He eventually took our order and when we finished, we stacked all the plates and saucers on top of one another and turned a coffee cup upside down on the top of this stack and put two pennies on the bottom of the up-turned cup. We took our bill and went to the cashier to pay and the manager came over and told us we were supposed to pay our waiter. We told him of the lousy service and the waiter's comment about waiting on us when he got time. We told him where we sat, paid our bill and left.
Why the two pennies on the top of this stack? We didn't want the waiter to think, we stiffed him, but we wanted to put in (tell him) our two cents worth about his service.
Good luck to you and yours.