The above example is good for us to know what we need to do to beat our costs, but it does not take into account that we don't want to merely bill enough to hit an imaginary number. For example, we never want to be consistently cheaper than competition. In fact, when possible, we want to sell the value of a repair. There should always be a maximum of return on a repair, regardless of whether we already hit the numbers we deem necessary to pay the bills.
Another random thought: How do most shops realistically track technician efficiency? We use Mitchell , so we use a time clock and my own head, and I input these numbers into the computer. It is an imperfect system, as in the real world time is lost to unbilled time all the time (putting air in tires, helping a lost parts guy, digging snow when its the season, etc.) It would seem to me a more logical financial metric would be total labor sold divided by labor cost per technician. I have heard the number for B techs of them producing at least three times their own cost. Because, in the end, we don't necessarily care how slow or fast a technician gets the work done. We care how much work we can sell out of that bay and how much selling that work costs us.
Last random thought: I have found that for certain jobs, you don't want to bill the labor out so high, so you compensate on the part when possible. Good examples are tires and brakes. Customers actually get offended if they see a high mount and balance charge, or 2.5 hours to do a brake job. While some customers will spend time shopping around parts prices, most understand everyone makes some money on them and they are less likely to fight about that. It is in these situations where I find the labor:parts ratio to be counter-productive, because you actually penalize yourself when you bump up the parts side.