I've had a play with the relationships cited in post #13 trying to end up with the two earlier relationships from first principles and starting with the presumed guidance of #3 .. with little success. Possibly my lack of imaginative manipulation so I need a little guidance regarding the goalposts, I guess.
I am presuming that each intends to achieve the same result, viz., a concatenation of a weight and an index .. in each case with the arm information represented as %MAC and stab trim setting units, respectively.
Now I see that scotiat claims some considerable competence in the subject in his cited website so, perhaps, I can ask him to give the horse a touch of the spur by setting out a simple example calculation for, say, a postulated light aircraft. That should sort out the confusion with terms and definitions very promptly.
As I suggested earlier, this is an important topic from a safety viewpoint and it is worth working through the detail for the benefit of
(a) me .. as we don't appear to use the technique in Australia and, at this point, I am a tad confused in that I appear to be unable to derive what ought to be a comparatively trivial engineering calculation
(b) the new chums .. as they possibly know less than I do about the weight control subject.