youre right about the lower legs. but its true in both the BWT squats and front squats, so i considered it negligible. as for the hips compared to the head, i actually made two measurements: one from slightly below my belly button (my 'center of mass') and another from my shoulder (where the barbell lies in a front squat). the measurements were the same. intuitively this makes sense, though, if youre doing a full ROM squat with an upright upper body (since two points on a line are equidistant relative to an axis no matter where they are in space so long as they are positioned with the same angle to the axis). im not saying i maintained perfect form throughout either of my sessions but i would consider the discrepancies in both comparable and, therefore, negligible.
regardless, the point i was trying to make was the fact that i was more effective in creating power with the BWT squats (and, therefore, justifying the 18-20 Tabata score before adding weight), which i think is undeniable...no matter what the exact numerical difference.