The subjects should not be severely underweight or obese, and they should be in good overall health. However, it is important not to write protocols too restrictive for the sole reason of being restrictive. To my knowledge, advantages of these very restrictive criteria have not been established. I had always thought of it is as an empirical covariate. I am not sure why you seem to recommend it as a predictor of fat-free body weight.
Correlation is the weakest form of modelling.
Vss is one of the few instances where it has been shown that body fat does not influence it to any clinically relevant degree. Body weight is useful in measuring metabolisms that can be expected to come into play when a drug is administered. Body weight cannot be linearly related to the PK of a treatment protocol. A nomogram for estimating TBW was published by Watson et al., Amer. This was created using a Caucasian population.