Flat rate manual

The HVAC business is going the same flat rate system. In our shop, we charged $110 an hour, time and material plus markup on parts. We averaged 15- 20 complaints per month about costs and were going broke. After going flat rate and increasing our rate to $210 per hour with upfront pricing our complaints about price dropped to 0. We did lose bottom feeders like Rodac but good riddance. We now actually make a profit, are able to pay better benefits and now do a better job because our techs are not under pressure to finish in a certain time. Plus they are not overbilling if he is given a price out of the book. He pays what he agrees to and not a penny more. Why would a price be OK before starting a job and not OK after. It makes my blood boil when some numbskull who has never run a business thinks he’s got all the answers and sees a crook behind every shadow.
P.S. Doesn’t the auto flat rate book give the mechanic the hours the book says? If it’s a two hour job and he does it in 45 minutes or 8 hours, doesn’t he get the same amount of money? Should Rodac get money back if the mechanic finishes quickly? Should he pay more if it takes longer?