All geometry relates to the geometry of Impact right...........so Id say the Elbow Plane is just as valid as any of the other so called "normal Planes". (To the best of my recollection)
Homer abandoned the right angles thing, it would seem. I get the logic of it , I think, but keep going back to the glossaries definition of the pivot. Which to me is Hands to Pivot. Its all about getting the Right Forearm on Plane, however that is done by the Torso and all.
That's really hard to wrap your head around without a Plane Board. There's really not much difference in degrees of Angle from an Elbow to a TSP. It's hard to grasp the alignment implications.