Noisy transmission......

[Supermario]

Hey all, looking for some input on proper motor pinion gear placement.
I test ran the motor prior to installing without the case installed. I placed the motor at various depth of mesh and noticed that it was obviously generated more noise with a tight mesh vs a loose fit.
I got it to where I was happy with noise vs gear mesh and not worried about breaking a tooth. But now with the case installed and the motor and tranny in the truck it sounds pretty nasty and I'm considering moving the pinion back a little more but keep in mind I have a 4:1 reduction installed but also plan to haul a fair sized dump trailer and lowboy in the future.
Do you guys just eyeball and listen to the tranny? How far away can I go without damage to gears?

I appreciate any input, thanks everyone.

Mario