[_paralyzed_]

Basically you go till tight, and then back off one shim. Again, a tight constant mesh causes unwanted heat. You want just half a hair of play.

The more in depth answer is that if you are using "X" size shims, and if 4 is too tight, 3 may be too loose. You may need to use 3 "X" shims and one "Y" for a perfect mesh. 3 ".2mm" shims and 1 ".1mm" shim, for example.

When assembled, if you turn the diff input, the outputs should almost instantly turn as well. Then turn the input in the opposite direction. If turning the input in the opposite direction immediately turns the outputs in the opposite direction, it is too tight. There should be just a hair of play when changing directions, like 5 minutes on an analog clock. Just a tick. Not tight, but not loose.

Get a bunch of different sized shims. In this hobby they are nice to have around. You could spend a whole weekend on diff shimming if you want to get real anal about it. It will definitely help, but in the long run, everything breaks.

Diff shimming is the same as setting a spur/pinion gear mesh. Don't bury it in the teeth, but also don't leave it sloppy.