The amount I leave depends on the service. If the service was ok but not great, I leave 15% rounded up to the nearest dollar. If the service was great I give at least 20%. But now if the service was great and our table caused some kind of grief for whatever reason (kids were demanding a lot or we kept asking for something) then I usually go a little above 20%. When the waiter or waitress is great and anticipates our needs, they also get the little more the 20%. I always round up after doing the percentage too. If 20% of the bill is $6.05, I give $7.