Men are obese at 25% and women at 32% (according to the chart I glanced at). For the sake of math let's just use an average male at like 5'8".  So at 5'8" an obese BMI (30) begins at 200lbs. So that's 25% bf with 50lbs fat. Add 400lbs to get to 600, that's 450lbs fat/ 600lbs total. That's 75% BF. Because women have higher natural BF%, I think at 600lbs (regardless of height or gender) an estimate of 75-80% is probably accurate.