Tracking your body-fat can be difficult, this equation combines correlations found using various methods of tracking body-fat such as DEXA, Bod Pod, Hydrostatic, Skin Fold, Bia Scales, Navy, and Visual Estimation to make an accurate reading based your strength levels.
If you've been doing my Super Saiyan Six program without any need for exercise substitution, you will know what your max effort lifts are on the Squat and Bench press (your 6+ lifts). Since how much you lift largely depends on how much muscle mass you have, this gives us a decent answer in tracking your body-fat levels.
This method only works using limit sets of the squat and the bench, so if you're injured and using work-around lifts, you slept/recovered poorly, or if you're lean enough that strength begins to drop then it will underestimate your true muscle mass since it's performance based. It will over estimate your body fat levels.
It also expects you to be honest. If you do half reps, bounce the weight, or go for very high rep PRs etc then it will artificially reduce your estimated body-fat levels. Don't cheat yourself.