Alright so hopefully I can explain this properly.
I'm replacing a fascia board where the corner can't be a straight cut. There is one board that spans the "bottom" of the roof and one that goes up the gable.
The long board on the bottom I cut to a inner 45deg bevel. This board, due to the structure of the house, does not sit at a perfect 90 with the roof plywood.
I figured, with the "bottom" board installed, I can use a t-bevel with the long metal side flush with the inside of the roof ply and the back end against the back of the "bottom" board I can get the 2nd angle I need to cut the gable with.
As such the gable will be cut at a 45 bevel and a miter of whatever the t-bevel yields.
Is this a correct approach? Better ways?