The only thing I find helpful about knowing bodyfat percentage is it helps plan the duration of the cut. I've had a full-body DEXA done at three different percentage bodyfat levels - end of cut, end of bulk, and in between.
I know roughly when delt veins emerge (for me, about 17%), when ab veins appear (closer to 15%)... so I take my weight at whatever point I'm at and guesstimate the bodyfat percentage, I know the approx LBM.
If I know the approx LBM and divide by (1-desired bodyfat%), I know what weight I need to hit. This tells me how many pounds I need to drop. I know from experience I don't like to drop faster than about a half to a third of a pound a week, so the number of pounds tells me my timeline.
For example, I'm currently 137 lbs, and about 17% bodyfat (possibly 16%). Call it 16.5%. This means I have about 114 lbs lean mass.
If I want to get to 10% bodyfat, I divide 114 by (1-10%)
114/0.9 = 127
So I assume (which may be foolish, but it's how I fly) that I neither gain nor lose lean mass because I diet and train very carefully - and proceed to diet slooowly down to 127 lbs.
If I'm more muscular than I think I am, well, I just look shredded a little earlier than I thought I would!
