Based on the dry fitting, with cylinder, would you like to see it a bit shorter or not necessary?
Not sure if shorter would be necessary, and it is surprissing the difference that the approx 1.6 inches difference in length makes. Perhaps slightly shorter could be useful but I am not sure how much as it would be anatomy/comfort dependent and my wife has not had it on enough to really determine that.
If you are having a custom plate made, then I recommend taking some carton/cardboard and cutting some templates in diff sizes and checking how they line up on your wife's back.
Just realize that their is an ideal fit of the plate when dry fit on a diver standing in a vertical position, which is sort of tossed out the window once a tank is mounted as the weight causes it to shift from that position.....things would re-approximate the ideal when in the water due to the reduced weight of the tank due to buoyancy.
-Z