On your receipt, there wll be suggested amount for tipping, based on percentage of the bill. I know one of the percentages they give is 18%. DH and I usually tip 20% for excellent service, which we did at Boma since we had a wonderful waitress. There was never one plate left on our table for more than a couple of minutes, drinks were automatically refilled for us, she checked on our boys to see how they liked everything, and she was very sweet to our boys and also to us of course. The service at Crystal Palace dinner buffet was lacking (worst service of our trip)...the waiter never cleared our plates and they piled up, we had to hunt him down for a drink refill, no smile at all on his face during the whole dinner, seemed bothered whenever we asked for something (which was not often at all...referring mainly to a drink refill). We tipped him 15%. Most people I know (who are not bad tippers) would not have tipped this guy more than 10%, if even that. What can I say? We tend to overtip...both DH and I have worked as servers before and we understand they live off their tips.
We also tipped 20% at Biergarten...that was the second best service of our trip!