Changes in overhead mean you'll need new flat rate manuals.
Hundreds, maybe thousands, of companies have switched to flat rate in the last few years. That change in the way service and repair companies did business was responsible for better customer service and improved profits for a significant segment of the industry.
Most of the new flat rate users received their flat rate manuals a few years ago. Remember when you first switched to flat rate pricing? You got new pricing books after making sure all the jobs your company performs were in the book. The prices were based on your (real) overhead and the profit margin you wanted.