The concept of charging book rate is fair for pricing purposes. But, do you want someone working on your car that has all his pay based on how quick he gets it done? IF all repairs were either 0% or 100% this might work.. When I picked blueberries as a kid in Seattle I got paid by the bushels or the pound of some such. Many kids even with their limited experience would try to fill the baskets with rocks or other things. It wasn't too hard for those paying to tell if they were getting blueberries.
In auto repair nothing is 100%. Sears went through hell a few years ago because they paid by commision. They found huge amounts of unneeded work being done.
In our shop comebacks are handled no charge. BUT, you must define comebacks. A repair that doesn't work or a part that breaks are that form of comeback. Bringing your car in because the check engine light is on again stands a reasonable chance of being a different problem, as any of the thousands of causes give the same indication (check engine light on).
Our policy in our shop is that the customer only pays for the parts and labor necessary to solve the problem one time (this doesn't mean we only charge for one part, many times the most complicated repairs are because one device has failed do to the poor performance of another). If we are wrong on a first attempt we will continue with the diagnosis. When we finally have the real repair accomplished we figure the cost as what it would cost for that one repair and either remove uneccesary parts or leave them but we won't charge for them or their removal.
We accept as a limit about three hours for any diagnosis even if it takes a week. A case with multiple problems might be the only exception. This of course is for electrical or drivability problems. It doesn't of course include disassembling a motor for estimating.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician