Thanks!

You cant, its best to be honest and err on the side of caution. If you look like the Michelin man, when you sit down but are not slouching, think around 40% to be conservative.

On a side note, I ran into a Canadian girl that was in the same field as me and we were talking about fat measurements (dont use the electric stuff, they rely on hydration, if you are a world class sprinter with about 8% but you havent had a drink all day it will list you as obese, its a waste of time. Best to use the mirror and be honest with yourself)

She said that they had a cadaver that was dontated for scientific use, they did the water/volume displacement thing and everything else they could think of. They they cut up the body and stripped off as much fat as they could find. The result? They were wayyyyyyyyyy off, like I said use the mirror, it never lies!