I am trying to figure rafter end angle cut for a 16'x16' roof that drops 18" in 16'

Divide the number of feet in run by number of inches in rise.

If u are making rafters that are running the full 16' from front to back your pitch will be 1 1/4''. This is virtually a flat roof.

If the ridge board is located at mid span then the roof would pitch to both opposite walls and be 2 1/2''.

If u are in a snow area the pitch is not enough to pass most building codes.

The lower the pitch the large the rafter size has to be.

Chuck