Have a look at this
http://dir-diver.com/en/equipment/backplate_adjustment.html

Remember that it does not actually matter if it comes off the
shoulders, it won't anyway, but the thing is held in place by the
waist and crotch straps - the shoulder straps are really only there to
carry it by. Feels different at first but you get used to it

Pete
diving 'at' melbourne 'dot' me 'dot' uk

Since the 'tightness' of the straps sets how high the plate hangs
that is what you adjust. The bottom of the plate hits the big bones
of your pelvis when the weight of the tanks out of the water are on it
and the crotch strap keeps it in rouighly the same position in the water.
Nothing is tight in the water.

If the straps start migrating out to the points of your shoulders consider
crossing the straps behind your neck (this is anethema to some purests
but do what works for you).

With the plate low the arm loops are big and getting in and out is easy
provided you get your shoulders against the plate.

HTH
nigelH

Having just set up a backplate, for the first time, and used one for the
second series of dives I will give my observations, being very clear
that they apply to me and others feel differently.

After getting advise from several users here I set mine up with crossed
straps and feel happy with that choice.

I also put in a quick extension (QE), I call it that not a QR because
the harness is not cut. One or two people have had problems with fixing
the attachment points of the QE and have had it slip. I think I have
solved that problem as my harness did not slip on the top attachment
point using two tryglides. The bottom attachment did slip twice using
one tryglide, however that is easy to fix as it does not carry much
load.

One further point which makes my setup very much a DIMW not a DIR one is
that as I really like the Buddy crotch strap setup I have incorporated
dual straps, intergrated weight pockets and trim weight pockets (for
keeping my snorkel when teaching) and DSMB reel in.

With just the harness & plate on, I made sure each loop was slack enough to
allow me to bend my arm and push the fingers on that side back through the
loop comfortably. This is fine for me - the loops don't need to be tight.

Im another that has bastardised an off the rack Halcyon system - I put a
"QE" (see post above) in on the right hand side, with toothed triglides top
and bottom, due to serious slippage that happened without them. I also used
an AP Valves clip due to strength & reliability (it was very cheap at a dive
show), it beds in quite nicely just where the strap starts to go under the
arm, and isn't uncomfortable.

Even in the open position, the QE is not a problem, I've dived with it open
and barely noticed. It does make it a lot easier to get out of the harness
though.

One other thing about your harness - it'll all stretch out a bit the first
time you put some strain on it, and the first time you get it in water it'll
swell up, then dry out, then everything moves around a little bit. After a
couple of dives it'll settle down, but I did end up tightening it all up a
couple of times before it was fine.

Jeff.