It depends greatly on your playing style. I play lots of double bass, 80%+ heel down. i positon my throne so that my thighs run parallel to the floor and my heel is on the base foot plate in a comfortable, natural/ neutral position.Most of the time, i'm playing at a fairly brisk pace, 160-200 bpm sixteenth singles, but i lose control and fall off tempo if i go too slow, 120 bpm or slower so i switch to what derrick pope calls a "steve smith" stroke (catch him on youtube explainig heel-toe method), heel up then flat foot straight down from the hip, so i definitely don't want to sit too low to be able to do this. As far as angle, i would say my knee is at a slightly greater-than-right angle, but not too far out.