Explanation in regards to "extendDistance" paragraph:

If a line is drawn perpendicular to the top point of the outer slant seam line, which is also perpendicular to the outer face of the 3D frustum, the intersection with the inner face will be offset from the end point of the inner slant seam line. And vice versa for the lower point of the slant seam line.

- Brian

Attempting Flow, no luck so far...