With AR10/15-style rifles, with free floating handguards, and typical rifles with free floating barrels rest the center of the handguard on the bag.
With rifles with handguards screwed onto a barrel lug, e.g., Rem 700, center the lug screw onto the center of the bag.
Benchrest shooters rests the fore-stock (handguard) on the bag pushed foreward until front of the forestock touches the stop on the front shooting rest.
Single-shot and Schuetzen shooters rest the barrel on the bag approximately 6" from the end. They have a way to test harmonics using talcum powder on octagon barrels and may work on some round barrels.
Stay away from the swivel studs.
It's all about barrel harmonics, rifle profile, shooting-style, and consistency. Plus, what works for you. By shooting-style I mean if it's a hunting rife you may want to rest you non-shooting hand on the bag while holding the forestock/handguard.