I don't even understand the question.
I have a feeling you might be thinking about excess skin during and after fat loss on a somewhat obese person. I'm not saying I have any knowledge about any kind of formula for that either, but like said above, a pound of fat lost is a pound of fat gone.
When I started my transformation, I eliminated 40lbs of true body fat over a period of about 9 months, and never had any problems with loose skin, either. That last 10lbs around the middle was absolutely the hardest, though.