Personally i get them to a point where i can't make them slip with moderate force by hand out of the car, then put them in the car and build it up as normal. you'll find they should slip fairly easy now with the wheels on.
I then run the diffs in by holding say a quarter throttle and holding one wheel at a time for approx 5 seconds. If the diff is slipping very easily, ill tighten the diff by a quarter turn and repeat until its close to not slipping and then ill tighten by say an 8th of a turn each time.
I consider mine tight enough when the slipper slips before the diff does, but obviously you need to be careful the slipper isnt too tight otherwise you'll end up breaking the crown gear like i did sunday
Main thing is your start loose and work it up so you dont break the rings!