to avoid loading up the threads on the hanger, it is much the best idea if you clamp the gear hangar between two track nuts on an M10x1 axle.
My favoured DIY method is to use two rear wheels, each with M10x1 threaded axles. The first is put in the frame with the sprockets on the left; this makes room for the other one to be bolted through the gear hanger and used as a tool/gauge for resetting it.
The first wheel makes sure that the dropout doesn't bend where you don't want it to (the clamping force of track nuts is about x3 more than a typical QR). It is relatively easy to see when the two wheels are parallel to one another and the gear hanger is properly aligned.
FWIW the tolerances specified by shimano are 0 to 0.5degrees, so 0.5 degree 'toe out' and 'negative camber' in horizontal and vertical planes respectively is tolerated. Any error in the reverse direction always degrades shifting, which is why it soon goes bad if the bike falls over.
0.5 degrees means there will be 5mm difference in the separation of the two wheels across a full (622mm) wheel rim diameter.
cheers