Thanks TSandM. That link is a great starting point. Follow those guidelines and go dive, adjust, dive etc. It will take a little while to "nail it"
The rule of thumb is that you should be ablt to touch the top of the backplate by reaching over the top of your shoulder, low and you need to tighten the harness.
Keep in mind that this plate position is so you can reach your valves when diving doubles. Single tanks can be moved relative to the plate much more easily than bolted on doubles.
Tightness of the harness is subjective to certain extent, definitely don't want it sloppy, but new BP&W divers often find that they will progressively tighten the harness, as they become more accustomed to using it, and confident that they can get in and out of it. That just means you need to trim it again, which is no big deal.
Good luck, enjoy
Tobin