You may also want to read the Houston Warehouse experiments
before finalizing a length. Different barrels and case capacities can cause two barrels to differ by 100 fps with the same load, so I wouldn't worry about the QL prediction difference. If I tweak QuickLOAD a little to give me your 2700 fps result from a 26" barrel, it says a 22.375" barrel should give you 2609 fps. That you actually get 2675 fps in your 22.375" Sako barrel tells me it likely has a tighter chamber or bore or both, so it's a fundamentally faster barrel. It's normal to have these differences.
For your bullet and load, figure to loose about but that if you cut the 26" barrel you have right now down to match that length, it's velocity would be closer to QuickLOAD's prediction. You should loose in the range of 0.9% to 1% per inch from 26" down to 18", landing on about 2470 fps at 18" if you cut down your 26" barrel and 2565 fps if you cut down your 23.375" barrel to 18". If you get yet another 26" gun and cut it to 18", you probably land in that range somewhere.