My personal take is to forget attaching your pack to your PC. Doing so makes it almost impossible for you to access the pack until you doff your PC, which is not a good idea if you are under a threat condition which mandates wearing a PC in the first place. Very problematic if riding in vehicles.
Bottom line: Run your pack, or hydro carrier with minimal shoulder straps, since the thing will be worn over your PC, and make sure it has a sternum strap which you can adjust to suit your needs.
There may be very uncommon situations when wearing a pack attached to one's PC make sense, but I think such situations rare. YMMV.
FWIW, I have yet to see a mil-issued pack that lacks shoulder straps. Some mil-issued hydro carriers lack shoulder straps, but IMHO this is a mistake.
Common sense says don't put your pack where you can't easily and rapidly get to its contents. YMMV