Ya but Jim, who likes all that figurin anyhow?
It's always nice to be able to put things up and scribe them, especially angled pieces.

In theory Jim's answer is correct, but in practice it could vary a bit. For most purposes however, you are fine using math. However my preference would be something along the lines of Thane's procedure, or just a straightforward shoot and scribe.

If you want to do it easy, do it Jim's ways, I guess is my best answer!