There is no real formula, but it can be calculated with a fair degree of precision. Multiply the radius of the bend you want to make by 6.28, then by degrees, bend and divide by 360. Divide once more by two, measure from the center of the pipe that far then set that mark at the front edge of the bending shoe. The center of the bend should be very close to the center of the pipe. If you use the NEC codebook to find the minimum radius of your bend, be aware that the figure given there is to the center of the pipe, not the edge, and correct accordingly.