Using your current weight and body fat, calculate your lean body mass this way:
Body Fat Mass = Body Weight x (% Body Fat)/100
Lean Body Mass = Body Weight - Body Fat Mass
For example, if you had a current body weight of 180 lbs and a body fat percentage of 15%, you would calculate
Body Fat Mass = 180 lbs x (15%)/100 = 27 lbs
and
Lean Body Mass = 180 lbs - 27 lbs = 153 lbs