There have been countess tests done and data posted on the internet where people have measured velocity, cut the barrel down 1" and repeated until they got to the legal minimum. The velocity difference is usually a lot less than most think, especially with common barrel lengths.
http://bulletin.accurateshooter.com/...s-with-length/
https://rifleshooter.com/2015/01/308...ld-medal-bthp/
The problem is that even from the same barrel length you will see very different speeds from different barrels. Around 25-50 fps is normal and I've seen some cases where there was 100fps difference between 2 different rifles with barrels the same length.
I have a 308 with a 20" barrel that shoots faster than either of my 22" rifles. And there is 20-30 fps difference between each of the ones with 22" barrels. My 22" Winchester 30-06 is consistently 60-90 fps faster than my 22" Remington 30-06.
Even if you were working with load data for a 26" barrel it wouldn't necessarily be accurate for YOUR 26" barrel.
In your case I'd take the speeds from the 18" barrel and add about 100 fps to it and run the numbers. That should be close enough to get you on paper with your shots. You can tweak things from there.