It's not so much trial and error Dennis, as it is using the PTG to understand what you are doing.
When I first got it I would do a job and then take measurements, usually at each stage, so primer, base and clear. Then I could compare to what it should have been, from the TDS. I usually found that, at almost every level, I was putting on too much. So, on the next job I'd work on reducing what I sprayed on. Up the pressure a bit for finer atomisation, dial back the fluid flow a touch, move a little faster. I stuck with one gun for this but then later started changing aircaps and fluid nozzles, followed by different guns to see what effects the changes were making. Learnt a lot about my guns and how to get the best results from them.
Repeated the process when I changed basecoats and clears and Wow! - that taught me a whole lot more. Using HS clear is a completely different proposition to using MS.
Now I do a series when I get a new gun, like the Kiwami I bought recently. You think you're spraying the same (like I would with a Prolite) but the thicknesses are completely different. I'm still dialling that new gun in.
The other thing a PTG is useful for is getting an idea of what previous repairs have been done to a car. Probably not as important if you're doing a complete and stripping to bare metal, but for repairs like I do, it's really handy to have an idea of what's under there.
Here's one example:
Customer came in asking about cracks in the paint (which you can see in the first picture). He'd had it fully resprayed by another shop which had a reasonably good reputation. I diagnosed cracking in bog just from a visual inspection but then went over the whole car and took lots of readings with the PTG to get a better understanding of what was under there and give him some idea of what work and costs might be involved. When he gave me the go ahead we stripped the whole lot back to bare metal and it was the filler that was failing - too thick in many places and either incorrectly applied (no epoxy) or incorrectly mixed. Had to fill only a few spots just to cover some fairly ordinary metal work and then epoxied and primed only before respraying base and clear. A big job, even on a Mini and matching the custom colour was fun.....not.
A few days after he picked it up he came back in to ask what we'd done to the engine. Apparently the car was performing noticeably better than it did before. Nothing done to the engine, just that the car was probably 40kg lighter! 
Seems like I ranted off topic a bit there, but the point of the story was that the paint thickness gauge got me the job, which took less than 2 weeks and paid around $15K.
