Yes, the blade bevel would be the backing angle if cutting from the top of the jack rafter face. That's what I meant by "face of the jack rafter set in the plane of the roof". Ditto for purlins, except the miter line is created by the complement of angle
P2.
Here's another
Jack Rafter and Purlin Compound Angle Formulas page. The "Sheathing Angle" referenced is Hawkindale 90°–
P2, complementary to the Jack Rafter Side Cut Angle.
