How to find fix ridge height and how to transfer measurements to rafters so it doesn't end up higher than the ridge board

Try Pitch to Angle - Angle to Pitch Converter (http://www.blocklayer.com/pitchangle.aspx)

Somewhere on that site, anyway.

There are 2 ways to do this.

1st and easiest =Take 2 scraps of wood the size of the rafter and copy the plumb(ridge) cut and the seat(wall) cut. Measure from ridge to outside edge of wall. You have copied the rafter.

2nd way= Measure from top of ridge to wall under ridge.
Measure from outside edge of wall , on wall, to edge of ridge.
Divide the number of inches from top of ridge to plumb spot on wall by the number of feet from outside wall to edge of ridge. This will give you pitch of roof.

The rafter length is determined by the number of feet of run and the pitch.

Chuck

https://www.askmehelpdesk.com/construction/cutting-roof-rafters-137845-2.html