“In my opinion, the hip belt should extend slightly above the iliac crest. For people who could lose a bit of weight, this can be a problem”
Belt location is ultimately dependent on both the design of the pack, and “what feels right” to the user. While it may seem that the rule of thumb indicates that most belts rest on or slightly above the iliac crest, there have been too many examples where this rule doesn’t apply.
Having fit MANY different pack designs while working at gear shops between the mid 80’s thru early 2k, I found that each company created their own rule of thumb – and was primarily based on the specific design of the pack.
In my recollection, some packs were specifically designed to have the belt either ride entirely Below or entirely ABOVE the iliac crest. (Lowe’s tended to ride higher while Kelty’s rode very low. And Mountainsmith’s lumbar packs all rode waay above.)
Regardless, it all goes back to the back manufacturer’s “intent”, and what the user believes is comfortable (which may ultimately change during their first couple hikes.)