I have used a Flat Rate incentive program for the last 30yrs successfully w/less than 1% comeback or repair complaints. But this only works if you have a religious & stiff quality control program in place to make sure it doesn't get "sloppy or out of control".
We do this thru training our techs w/weekly video training programs, weekly shop status meetings which include round table discussions between mgmnt & tech's, spot checking the jobs with random QC tests drives/inspections from svc mgr & a follow-up call to the customer w/i days of there p/u.
These procedures have always taken a little extra time & commitment, but the results are great! The tech's are happy & profitable, as the owner--I'm happy & profitable, the shop has a good vibe & flow & the most important thing is the customers are happy & actually overwhelmed that a Auto Shop would follow-up with them to make sure they're satisfied with the work & performance of their vehicle.
Thats my best suggestion of what I know works with the proper management & implementation. Good Luck, Rod