Please correct me if I am wrong (king of the mill), but I think the numerical distance is not important; it's the angle between the line from pin to PAP and the line from pin to MB (which used to be CG, before the MB became a more accurate gauge).
I believe we started using the numerical distance of the MB as a convenience, but that is only accurate for the average PAP distance; it depends (again, I think) on the actual PAP and the angles involved.
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