Bisecting the Negative Cast for Orthotic Manufacturing

I apologize for most of my readers that this means little, but this bisection line on the back of a negative cast to me is crucial in designing orthotic devices. To complicate matters worse, the bisection line must be "tangent to the curve on the lateral side of the heel", whatever that means. I know what it means of course. But, when you are learning this stuff, it can be difficult, and sometimes impossible to learn. The lateral side of the heel is the opposite side from the level, and looks fairly straight. When you use Root Biomechanics, 1 degree changes are important, so a heel bisection 2 or 3 degrees off, can be disasterous. With the Inverted Orthotic Technique, with a rough estimate of 5 degrees of cast inversion to 1 degree foot correction, the exact bisection is not as crucial.