Thank you guys.
Gabe, The choil is the very back of the edge where it sweeps up to the ricasso. Some knives dont have a choil, so dont be confused. The pinch is just that, I pinch it with the hammer and anvil to pull it down and define the back limits of the blade edge. This establishes the length of the blade somewhat. Since the remainder of the edge is yet to be beveled, you have to allow for the length to grow. So it's an estimation based on experience. It will work out about 25% longer after the edge is pulled down and the blade gets straight.