Renner has some great free technical documents, including a good one on tuning lever design: See number 8
The angle is measured as the difference from 90 degrees, which would be the perfect angle (a "zero-degree head"). The only reason we don't use zero is so that the handle clears the case on an upright and struts on a grand (especially Mason & Hamlin).
Another way to get clearance is with a longer tip, but you lose the feel of the pin, and it's easier to slip off.
So you choose a combination of head angle and tip that works for you. Schaff still sells a ten-degree head, if you ask, and I've wanted to try it.
They also make a thin-wall extension tip that has an end shaped like a tuning pin, so it fits right into your existing tip without changing heads. It's a fast way to gain extra clearance when needed.