Yeah you can't figure straight lean muscle mass easily. The proper term is Lean BODY Mass...which is how much you weigh (skin, bones, internal organs, blood, and muscle tissue), minus the fat. they have averages based on height and frame to determine how much bones weigh...its 10-20 pounds. you only have so much blood, so that's not hard to remove.
but internal organ weight will always be an estimate unless we have a volunteer for an autopsy...
really, LBM is good enough. you can gauge how much lean mass you have gained from a bulking cycle by knowing weight and bodyfat...and vice versa for a leaning out to figure fat loss (and possible muscle loss if you lean too quickly)