If you shingle before gable board is on, How do you keep it from curling?
Well I think you mean the fascia going up the rake? I’d compensate for the thickness of the fascia add 1" to that if your not gonna use dripedge. And I always run an “edger” up my sides I like the gaf pro starter strips. This should give you a firm edge till they put the fascia board on
I always precut for an exact fit, or if I’m going to cut the rake afterwards, I precut so it never sticks out more than an inch past desired finish. It’s never curled on mine.