I go by the Q-angle, but judges just look to see if you hips are lower than your knees. My box squat for comp is about 13.5 inches, but I've been training to 12'. The distance from the vertex of the angle (the underside of my knee) to the floor is 18 inches. Low box squats to 8" will be added later. For those that don't have the hamstring strength to lower slowly to that depth, I agree with aurelius on taking it lighter at first. The downward compression caused by losing control, and FORM, is not worth the risk. Build the hams up first, and work on a low squat later.