My apologies friends, I forgot that it is different if you put the square on the bottom,
The correct angle to cut is 43 degrees NOT 47. You have to subtract 47 degrees from 90 degrees to get correct angle if you use the speed square this way.
You can do this from the top of the rafter, but if you measure from the bottom, you need to subtract your finding from 90





In this video, Mr. Kongable discusses how to use a speed square to find the angle of a rafter with it is unknown.



music is copyright free:
Wanderlust – Scott Buckley (No Copyright Music)
https://www.youtube.com/watch?v=s3fEb.