found this site...

[jennifer86]

once you get your lean body mass (weight of skin bones and organs etc) you can add your ideal percentage of fat (eg 12% for girls) and thats what your weight should be.

im 5ft 8. my lean body mass is 109lb (i have a small frame) and so adding 12% to that i should weigh 122lb. however sites have said that i shouldnt weigh less than 126 so ill say my goal is 126 and see where i go from there.

 

[Halo1]

Im not sure if that 10%-12% range for girls was ment to be the 'ideal' I got the impression it was more 'this is what you must have to keep you alive'. I calculated my lean body mass and added that 12% and it came out at only 120 lbs, that would give my a BMI of only 19 and that seems pretty low too me...
Then again everyone is differant and im no expert...