In truth, there's no true way of comparing. Ppg doesn't work either as there were more draws when wins were worth 2 points.
I never meant this as a definitive comparison, but useful enough as a guide and talking point. Very few would suggest, for example, that Neil Harris has a good home record. I never realised until I did this how poorly Jimmy Scoular's teams performed away from home. The data's good enough to make those suggestions.