For the rafter to ridge (called a plumb cut), it's simply a 90 deg to your rafter angle.

For the birds mouth

1. Draw a plumb on the rafter where the seat cut starts.

2. Set the "Diamond" over the plumb cut line with the heel of the square against the rafter edge.

3. Scribe the 3 1/2" line and make the triangular cut.

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