Hey Aardvark you say you build your custom models (that aren't C.U.P. or D.o.A.F.) to canon proportions. What advice you would give to get those canon proportions? Do you just compare a clothed version of the character to the nude body your going to give it? What about the case of character that have very bulky clothing?
Try to find the tightest-fitting outfit for them they can. Then tear your hair out when you realize the character designers / artists didn't actually keep their proportions consistent between outfits.
Similarly, use cues from what tight-fitting outfits you can. For example, a character wearing sweatpants and a bulky coat - the coat doesn't give you much information, but the tight-fitting sweatpants give a lot of cues to their physique. Are their legs thin and wiry? Then they probably have a thin physique. Thick and curvaceous? Then they probably have a thicker frame.
Basically a combination of sleuthing, deduction, and good-ole-fashioned intuition.