Hi Bingo,
I'm assuming that you really want to determine a realistic goal weight for yourself based upon the amount of lean body mass (bones, tissue, organs, muscle) that you currently have. To do this you will need to get an estimate of your current body fat percentage.
As Mel pointed out most techniques are only estimates in nature and as a rule we really look at the overall trend of lowering. I have found this little device helpful.
http://www.amazon.com/Omron-HBF-306-.../dp/B00006WNPU
Okay. Back to determining your goal weight. I'll just through in an example here for illustration:
Julie weighs 131 lbs and is at 22%bodyfat according to her morning weight in. She would like to reduce her bodyfat percentage to 18%
131*22=28.82 lbs of bodyfat
131-28.82=102.18 of lean body mass
102.18/(1-.18)=102.18/.82=124.61
Her theoretical goal weight at 18% bodyfat would be about 124 to 125 pounds.
Again. Don't get to hung up on numbers. Look at downward trends. I think it is helpful to get an idea of your current bodyfat % and lean body mass in a ball park perspective to kind of know if your goals are realistic.
In my case I thought at 5'3 my goal should be 110 to 115lbs when I began.What I didn't know in the beginning is how much muscle I would be able to retain and build along the way. After finding my current LBM and knowing my goal of 18 to 19% bodyfat, that low of a weight would only be possible if I intentionally lost muscle mass. Since muscle mass enhances our metabolic processes, this would be counterproductive in the long run and increase my chances of gaining weight back due to a weakened or inhibited metabolism.
Good questions. Again. Look at trends. Keep trying on pants that are just a little snug, do a waist to hip ratio. There are all kinds of ways to measure our progress.