Sorry.... You want 0.3 INSTEAD of 0.2mm, not on top of 0.2mm. The shimming inside the diff, with brand new gears and case, is 0.6mm total. Anything more than that and you end up with parasitic drag. (At least, in the 4 that I have built, those were the tolerances)
If your gears are already worn from slipping (the clicking noise people get inside their diffs) then you might be able to shim to 0.8mm or even 1mm total....but with new gears, you want 0.3mm per side to get 0.6mm of shimming.