BW,
It should depend on both your jet pack and backplate. I would put my vest and backplate on and then get a helper.
Here's what I did. The jet pack should sit so that the curved part at the top of the pack (that is right up against the backplate) is 1.5-2 inches (roughly) below the top of the backplate collar when mounted.
Then, the hooks should be on the pack so they align at the top with the slots in your backplate and are no lower than 1-1.5 inches down when attached to your straps. How you get the hooks mounted there is the trick and will almost certainly take a helper or mannequin. Basically find a way to center and hold the pack in place at the desired height as I described. Then put some painters tape on the part of the hooks with the holes and place the hooks on the pack so the tops align with your backplate slots or just slightly down. Make sure the horizontal edge on the top is parallel to the slots. I actually put mine right below the slots, figuring the weight of the pack would pull my harness straps down just slightly and it does.
Let me know if I can be of any more help!
Chris