Damn, I knew I shouldn't have clicked on this thread.
From my experience I can tell you that the hipbone was the furthest forward point, not necessarily the peak, of the bone. Line up a plane 90 degrees straight off of that point and that's basically where the measurement is checked from. To be safe, and to avoid being poked, just use the outer seam of your pants or the pocket as an index point. Roughly 3 o'clock on your waist. It's a tough and subjective rule to call and enforce, so just err on the side of caution. Or do what I did. I bought an Open gun this afternoon.
Or you could push your luck and come challenge my title. After all, I am the defending Open Champion (at the Limited/Production Nationals).