The angle is a byproduct of drawing a line to connect one corner of the forearm to another point at the wrist.

Your forearm to your wrist is a truncated cone. The vambrace is a cylinder until you cut it.

One end of the vambrace is the circumference you want for your forearm. The other side of the vambrace should be whatever circumference you want for your wrist (with gloves on, etc.).

So if one end is 11" and the other end is 8" that's a difference of 3"

So assuming you want to make 2 symmetrical halves that means you have 4 total cuts to make (top half has 2, bottom half has 2). So divide 3" (the difference between 11" and 8" in this example, your number will be different) by 4 total cuts. That gets you .75. So 3/4 of one inch.

So on each half, you put a dot at the corner (or 0" from the corner) on your forearm side. Then on the other end, the wrist end, you measure in 3/4" from the corner. Now draw a line connecting the two dots. That's the "angle."