According to "textbook", where should power pull buckle rest when a waist belt is tightened for optimal performance? I see some pics with the buckle centered over the belly button while others rest lower.
As jmez states above- the ideal location for most users is to have the top of the iliac crest (bony ridge of hip bone) on your sides centered in the middle of the waistbelt. So then where ever the buckle ends up being in relation to that is where it ends up.
Some like a little lower or above this; but this is a good general guide. That is what is great about the Kifaru waist belt's design- as the belt is both wide enough and the right thickness ( not too thick) and it's pliability helps to have it wrap around and cradle your hip bones with just the right pressure.
And then you couple all that with the power pull feature and the adjustability of the delta staps- and it gives you a very secure fitting of the load to your hips. It just seems to lock the load right where you want most of it (the hips) and helps to keep it there.
And then the great non slipping lumbar pad in the back of the waistbelt helps to complete the package. Just a great design and IMO one of the very best features (their waistbelts) of the Kifaru packs line.