Personal choice (from the pins) is around mid-thigh. Want to focus more on the actual upper body pull and keep the leg/hip/glute drive to a minimum. I do do a slight knee dip at the start of each rep. If wanting more of a complete Hi-pull motion, than I start from a bit below the knees in a PR and/or boxes and platforms. Seen a few lifters do Hi-pulls, or cleans, on their knees for more of an isolation effect on the actual pull.
A totally different affect is when using rubber bumper plates for Hi-pulls or deadlifts/SLDL's. Going to be using much more weight than you would think possible. The bounce can make all the difference in the world when seeking power and mass. Timing can also enter into this.
If wanting to really go old school, than suggest Hopper/plank Hi-pulls, Dl's or SLDL. A real mass builder, and all done in higher reps (12-20 reps).
Far as BB'ing goes, might suggest the old Pre-exhaust system. Start with a set of BB shrugs followed with a set of Hi-pulls. There is no rest between each set. After the Hi-pull rest 60-90 seconds and repeat the cycle. Pre-exhaust can be applied to just about any buscle group. A very effected way is to do lateral raises followed by presses behind the neck to hit delts hard..as another example.
Good Luck.