First, get the pack and, run it under load for a week or so getting the bugs worked out. Make sure the harness is adjusted correctly first. It might take a bit of fooling around doing this. Just start packing weight in it. Because, as with any pack, there is close enough to work good and, there is perfect. Which is one of the reasons I went with Kifaru. Micro torso adjustment for each side. The 1/2 inch slots and such found on some packs work ok with 40-50 pounds. Over that, if it's not perfect, you'll start to see it.
In my experience, too long and the pack won't settle in the curve of your back. It will slip over time. If you are having to adjust the belt up and, having to re-cinch it doing training hikes, this is your problem. Too short and, there will be too much weight on the shoulders not allowing the pack to settle in the curve of your back. So, fool with it. There is no time line. Just get it as right as you can.
Only after extensive and exhaustive efforts to ensure the harness is the right length, would I go to altering the curve on the stays. And, I' wouldn't do that without extensive consultation with someone fro Kifaru.