Good points and there’s many contributors to variation/precision above. However, I provided 5 equations above with 4 inputs among them. So the user can at least compare the relative dispersion among the results from the 5 equations. To your point, the accuracy given the gauge/operator error is not addressed, and propagation of error is outside the scope of my post. One can run a DEXA scan/other “measurement” if they want to get some sense of accuracy. IMO almost no one would use or follow any dissertation I would write on propagation of error.
[To prove my point, let’s see how many times the link in the previous sentence gets clicked.]
In fact in my profession almost no one does this properly, and this is among trained scientists/engineers. Hence, I stay in business and business is good.
Short of killing the subject, the best we can do to get a sense of accuracy is to triangulate with DEXA, BodPod, and caliper methods + some underwater methods. Again, I save this for another time.