I think the idea of confirming the gratuity with the delivery person is a good idea because it can help take away that feeling of stiffing someone - even though you aren't. Also, the full amount of the gratuity often doesn't go to the delivery person, so I usually figure they are receiving about 15%. If the service is very good I'll add a couple of dollars, maybe a litle more if the bill is very high, to get the server to the 18-20% range.