Tack a scrap board in place and either scribe against your eave side fascia or rafter tail or use a short "torpedo" level to get your true plumb cut. This should be the same angle as at the ridge cut. You could also use a simple sliding T-bevel against the fascia. Look up "roof framing 101".
What pitch are you trying to achieve? Fascia boards don't get mitered cuts. the rake ( on the gabble ends) will be set to the same pitch as the roof. Get a speed square, mark your lines and cut it with the miter saw. Use your speed square on the sheathing of the roof and bring your fascia board up against the speed square and nail it. Drip edge will go over the fascia and sheathing to cover factory edge. Unless you have some fancy woodwork, I've never heard of anyone mitering the fascia to match the pitch of the roof.