estimation method, either `green` (default), `boer`, `james`, `hume`
digits
round to number of digits
Details
Note: technically not the same as fat-free mass, although difference is small.
References:
`green`: Green and Duffull. Clin Pharmacol Ther 2002;
`james`: Absalom AR et al. Br J Anaesth 2009; 103:26-37.
James W. Research on obesity. London: Her Majesty's Stationary Office, 1976.
`hume` : Hume R et al. J Clin Pathol. 1966 Jul; 19(4):389-91.
`boer` : Boer P et al. Am J Physiol 1984; 247: F632-5