I asked HMG a query via email but they are a bit slow in responding, so I'll ask it here too....
I have seen HMG's recommended pack fit procedure, but I was wondering about back panel lengths between pack sizes. I have seen a small Southwest pack, a medium Ice pack, and medium Porter 4400 in person. The small SW had a distance of 18" from the shoulder strap attachment point to the bottom of the hip belt/lumbar pad. The same measurement on the medium Porter was 20". Is it safe to assume this back panel length measurement increases by 2" as you go up in pack size (i.e., the large would be 22" and the tall would be 24")? Does someone have a size large or tall handy they could hold a tape measure to? The stock hip belts and lumbar pads are all ~4" tall, I assume.
Was there an upcharge for customization (an extra strap sewn on, or making a modular hip belt-lumbar interface)?