I also played around with knees. When I started playing with lego we had the old minifigures that if you wanted them to sit, you just moved their legs a stud forward. So I wanted to recreate a similar effect for the modern minifigs. I wanted to keep the bum-holes in the same relative position to the torso which meant the length of the thigh is based on the back-of-calf to centre of bum-hole distance being 4mm (when sitting). The vertical distance (when sitting) from bottom-of-thigh to sole-of-foot is 1 brick (9.6mm). Finally, when standing, the new knees-figure is exactly 1 brick taller than normal.