Not really; indeed, my point is that body metrics are surprisingly difficult to model given the wide variances.

You could probably get closer to an accurate model by identifying the fixed weight of an adult's skin/bones/organs. Going from critically emaciated to fit would mostly entail adding muscle, which is denser than fat.

From there, the proportion of added muscle v. added fat when increasing waist circumference would be the deciding factor in accurately estimating weight; that proportion is highly dependent on the individual and one's level of daily activity.