To follow up, agree that compact dead weight isn't a good pack preview tool. Luckily our REI has a barrel of sandbags (no monkeys) and pillows that combined can decently approximate a full load with the weight distributed where I'd typically concentrate it. Confess to being too darn lazy to lug my own stuff to a store.

At home, jugs of water and random clothing work well.

Cheers,
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--Rick