I am building a very small lean-to style shed and was cutting rafters today. I cut all the birds mouths just fine through trial and error but I have a question about one dimension of the joint: I know how to determine the angle of the short side (roof pitch), and the length of the joint (sill width), and how to use a speed square to lay it out, but I don't know how to determine the correct length for the short side (see below).
birdsmouth.jpg
Is there a correct measurement here? Or is it more important that they're all the same. I know it shouldn't be more than 1/3 the width of the rafter material.
Thanks.