I agree with level3. It's also important to remember that people have different torso heights so it will also vary depending on your size. You want the harness to rest between your shoulders and in the small of your back. I found this to be more comfortable. I originally had it higher which put a lot of stress on my shoulders.
Once you find a comfortable fit for the harness you need to mark the holes in the vest. First put the vest on as best you can and have someone mark the holes for the metal bar. Then, remove the vest and harness and put on just the vest and back plate. Putting the back plate on, get someone to mark the holes for the straps, knowing that they only should stick out a few cm. You could do this while having the harness on, but I found this made a better mark as the metal bar puffs out the vest unless the bar holes are cut.
Since the weight of the jetpack is primarily taken by the bar that was to be my starting point. After cutting the holes for both the bar and straps, I put the vest and backplate on, putting the bar and straps in their proper places (the metal bar should be between the bottom gap in the backplate). Not worried about strap length just yet, this can always be changed.
You need lots of help for this part. Get someone to put your jetpack and position it along your harness. Most images of the jet packs I've seen have the bottom hooks flush with the bottom of the jetpack so I used that as a reference. Be aware of how high the rocket is going to be (you don't want it over your head). For me, the bottom hooks worked fine sitting flush on the bottom, so I drilled them in. Then, again, placing the jetpack on the bar (with help) you mark the locations for the upper hooks. Using a mirror, help your squire position and mark the distance your jetpack will be from your back.
Once marked, drill in the upper hooks, shorten or lengthen straps as necessary, and thank your squire
It's definitely a lengthy process and the hardest part to get fitted right for the costume. If it doesn't fit right for you, you will get sore much easier, so make sure the harness is comfortable!!