Currently I weigh somewhere in the region of 17 stone (108 kg) and I'm about 5'6" (1.68 m) which makes me 38.3 BMI, my clever scales say I'm somewhere between 47 and 50% fat, I'm female and I'm 30.
Guidelines say that people should be between 20 and 25 BMI, and women my age should have between 21 and 33% bodyfat. So by BMI I should be between 8 stone 12 (56.5 kg) and 11 stone 1 (70.5 kg). At my thinnest I was just under that upper limit.
Let's assume that I'm at the higher of those two fat percentages. That means I currently have 50% lean mass (8 stone 7, 54kg). If I lose only fat, then 21% bodyfat would be at 10 stone 11 (68.5 kg, 24.3 BMI) and 33% bodyfat would be at 12 stone 10 (80.5 kg, 28.5 BMI). 20 BMI would leave me with only 4.5% bodyfat.
Of course you *don't* lose only fat when losing weight, and that still leaves me needed to lose over 4 stone to be in a healthy range, but it's definitely food for thought.