[DOCW3]

to have zero shift on the turned shoulder plane is it necessary to have a flat shoulder turn?

Looking beyond your question and reflecting on the struggles I had with the "plane" subject, my emphasis would be on the statement in 10-13-D. "the Right Shoulder moves toward Impact precisely on the preselected Downstroke Clubshaft Plane." For a zero shift, this means to me that the right shoulder must reach in the turn back and move down the address shaft plane. The flatter the address shaft plane the farther the shoulder must turn back which is challenging for the less flexible. If the right shoulder can't reach the "plane" in the move back, a steeper address clubshaft angle must be selected. Although Mr. Kelley appeared to prefer steeper planes, there seems to be a preference for planes closer to the elbow. The photos are educational and I will leave it to you to conclude the impact of a steep shoulder turn. Or, I stand receptive to calibration.

DRW