but with the 72s i would move them to the lower d ring once they felt floaty then feel like i needed to move them back up because they hung below my leg then floaty again and so forth...
This isn't making a lot of sense to me.
I can't think of any SM cylinder that has a heavy tail that gets light then heavy again as you use the gas up. Unless there's something about how your neck bungies are yanking on the other end which seems extremely unlikely.