I was taught that the top of the hip belt should be around 2" - 5cm - above your hip bone.
Pull the shoulder straps untill they are "snug", but not 'tight, tight'. Raise your arms above your head as if streching, if the belt raises above your hip bones it might be too short, depending on your intended use. Scrambling, climbing, you would be better with a slightly longer back.
If you have them, pull load adjusters untill just snug, most of the weight should now be on your hips. If not try again, and 'fiddle' about to you think it's right. Now jump about, if the pack moves around too much, try adjusting again. Jump about again, if pack still moves around too much, try another pack.
Buy the one that moves about the least, if at all.
RE. The sternum strap, it should be 'just' tight enough too keep shoulder straps in place, not moving them closer together.