Yep, that's right. Denominated in some fixed weight and any fixed fat percentage the delta is the same (e.g., if my weight is exactly 150lb or 200lb or 250lb an

Yep, that's right. Denominated in some fixed weight and any fixed fat percentage the delta is the same (e.g., if my weight is exactly 150lb or 200lb or 250lb and, at some concrete weight, I evaluate my real BMI at X and my naive BMI at Y, then the delta Y-X is independent of my target 20%. But my target is to waffle around 20% or so on average.