I haven't watched mechbox.com...so apologies if the following is in the vids.
1. Google "Devgru Shim Guide"...one of the nicest write ups out there. Excellent pics.
2. There's no magic formula...what thickness shim that works on one setup may not (probably won't) work for another.
3. Degrease (just wipe it down clean with a kleenex) the mechbox, gears, shims and bushings...makes swapping shims a lot easier.
4. Use a sharp knife/xacto to pick "stuck" shims off of gears
5. Normally remove the cylinder/piston/tappet/nozzle/spring/spring guide while shimming. Consider also removing the cutoff lever/selector plate/trigger/(anything else that's in the way) as well....it makes it easier to "feel" both sides of the mechbox/bushings/axels.
6. As you're testing...tighten at least 4 screws evenly around the mechbox. With some not so great parts...snugging up the shell changes the clearance dramatically. Final test by tightening up all the screws
7. When you think you've got it, grease up the gears, don't put everything back in, tighten up all the screws. Spin the sector gear by hand (or by reattaching the motor (you've got to put the selector/cutoff/tigger back in). Open it back up, remove the gears and clean out any excess grease that was flung off the gears.
I found the above helpful over time with various mechboxes...best of luck!