Generally, I start with 15%. If it is something that is worthwhile I go to 25%. Something really good goes up to 40%. After that it is no longer a tip and a service charge which you feel is something you need to add to the service fee.
FUNK, did not even realize you had reentered the Master Control Room here at the Rx......careful not to touch that Big Red button to your left unless at least two other Mods turn their keys at same instant