Guys, I've been reading some conflicting guides on the best starting position for rack pulls. Some say to start below the knees, others say at the knee and some above it. What's the proper method?

I've tried both below and at the knee. The higher the bar, the easier it is to move some really heavy weight. My goal is to thicken my back, not just to move a lot of weight, so what's the best form? Should I worry about the bar position in relationship to my knees or the angle of my back?