Well I do them from 2 different heights.. One is with the bar about 2 inches below my knee caps and the other is from the top of my knee caps. This is the area that I stall out in when deadlifting.
Hey it really depends where your weakpoint is. But for a me a good place to start is at knee level. This aids in removing the legs from the deadlift motion, and allow you to target your back better.