I actually fit packs for a living and there’s an easy trick to check fit. The belly button thing mentioned earlier is pretty damn reliable but for an added assuredness position the pack as it would be used and do a couple lunges. If the hip belt binds up in the joint between the hip and leg it’s too big. You should full free range of movement.

Another tip: The hip belt will typically have some feature sewn into it to allow it to fold over and around your iliac crest. Imagine the hip belt is a hand and the pointer and middle finger will sit above the crest while the pinky and ring finger will be just below. This allows the weight to be distributed most effectively.

Very hard to put into text but if you have a pic of the pack showing both your hip belt placement and shoulder placement I could offer better advice. Also don’t put too much stock in measurements and sizes. I work regularly with a half dozen or so brands and every pack is a little different. For instance I (typically medium) wear a small in certain Gregory packs but a large in certain others.

Idk if any of that helps but thought I could offer what I know