Why PTG is a great investment. Most car manufacturers will only allow roughly 25% of the total clear coat thickness removed without long term durability problems "Premature Clear coat Failure" being an issue. If you started off with 50 microns(2mils) of clear coat, you would only be able to remove 25% or 12.5 microns(.5mils) without voiding the paint warranty and possibly having problems in that repaired area down the road.
I am going to the extreme here with regard to sanding, compounding, and polishing but why not for the sake of this thread. The only way you can ensure or track that is if you have a PTG. First time you burn through a panel and it costs you big $$$ most likely, you'll have a pissed off customer, a hefty bill, plus lost money on that detail service. Even if you have insurance you still have a deductible.
There are gauges out there Deflesko for instance that can measure the different layers of paint, and will measure coating thickness over plastics, composites, ferrous and nonferrous metals. BUT they're in the $4000+ range.