we always left enough extra to meet 20%, usually it was under $5 more so we just left a$5 ( we split one voucher between us). this is one thing i noticed.. every waiter we had seemed shocked when we did so. just my opinion but i am already getting a steal with the food, I don't need to be cheap with the service person too! I firmly believe many do not tip additional and the one snotty waiter we had at boma( called them food stamps , gave us lousy service) was totally embarassed ( I think because of what a creep he had been) when my husband gave him the extra...,hubby is nicer that me i would have taken the prepaid 15% "back" if I could have there but maybe he won't be such a CREEP to the next family he waits on with them)
also at pepper market the service charge works out to something like a 3-5% tip for the server (we asked the manager there) so they really should be tipped more since they did as much as a buffet service would have