Base of Skull (if that is what Homer used as definition, this where all head motion on it own originates from)
Base of Neck (difference may be minor depending on the head orientaion which means it would only change the position on the ground by less than a few inches)
Interesting, this really coming down to an attempt to define a Fix Point in space that is sort of (words choosen carefully) independent of the body, but yet required percision to support proper and correct geometric alignments....
I think you are really on the right track here with your illustration of the Pivot Swing Center Tripod. It is the first time anything like this has been done, and I commend you for it.
Based on the logic of our prior posts, please re-label the Horizontal Hinge Pin to Pivot Center Hinge Pin and repost. Thanks!
PS I really think that Homer's incubator ultimately would have given him the Pivot Stroke Center Tripod (for consistency with the Swing and Hit differentiation) but, oh well...