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In a pinch I resorted to some good old geometry, which was easy enough but did require things like knowing how to calculate angles etc.
As far as your ridge connection that has to do with the square & level of the building if you are cutting the commons the same.
I think the best bet for you would be to go and buy a Swanson Speed Square with the "blue book"
As far as the math goes there are construction calcualtors that all you do is put in the run & rise it will tell you the rafter length, angles, steps, etc.
On my site as well there are a few projects that go through the roof framing step by step with images.
Alter Eagle Construction & Design