Aren’t you suppose to secure the framing to structure??

Isn't the best way use a rafter book or look it up on a computer. A rafter book gives the length off the rafter by by the span.. If you deduct the thickness of the ridge then their is no need to shorten the rafter at the top. to mark the birds mouth if say you want 3 1/2''' just draw a plumb line at the seat and slide a 2x4 down this line until it touches the bottom.. Also if you use a book and deduct the with of the ridge you can look under hip or valley and use the span of the common and it will give you the exact length off the hip or valley no shortening Also when framing the hip it does not nail to the ridge

Straight to the point! Great explanation