Are roof angles necessarily standard? I ask because I'm going to be working
on the rafters and need to cut an angle piece to fit in beside the 2x8
holding up the roof.

I measured it out and came out with 9 inches over, 2.5 inches up which
should equal ~15.5 degrees for the roof angle. I'm not interested in
perfection, it just has to fit reasonably, but if I can get it exact it sure
would be nice. However 15.5 deg doesn't match a 3 in 12 or 4 in 12 pitch
more like 3.5 in 12. That's why I'm asking if roof angles are standard, or
at least WERE standard in the 1960's.

Just for clarification, my attic is ridgepole construction, 2x8 supports for
the roof, every other truss has a 1/2 x 10 board connecting the 2x8
supports. Weird construction, doesn't look sturdy at all in my eyes.