Heber, What brand is the pack? You should be able to call or email the manufacturer to get that info.

I would do what you have sugggested. If you measure your own torso length and compare that to your pack, that should give you an idea as to how the manufacturer sized the pack. Granted, they may not all do it the same way, but that might help.

If the pack is adjustable, you'll have a bit of leeway, especially if she is still growing.