There are lots of factors and it is almost impossible to say for sure. Most common barrel lengths are 20-24". With anything in between those ranges I've found 15-25 fps/inch to be most accurate. Actually the most accurate method I've found, (and it is still not great), is to predict velocity change to be somewhere between .5% up to 1% for each inch of barrel length change.
The only numbers that really mean anything when doing research is to find someone who has chronographed a load from a longer barrel, cut the barrel, then chronographed it again with the same ammo. There are way too many other factors when you compare velocities from different guns. Around 50 fps difference between guns with equal barrel lengths with the same ammo is common and I've seen as much as 130 fps difference. I've also seen 20" guns shoot faster than 22" guns and 22" guns shoot faster than 24" guns.
As you start cuttting barrels shorter the velocity drops off in larger increments. A barrel cut from 20" down to 16" will likely lose a lot more than when the same barrel was cut from 24" down to 20". The chambering matters too. Rounds like the 308 are more efficient than magnum rounds, but no matter how short you cut them, the magnums will always be faster from equal barrel lenghts. I often read incorrect comments like " if I were going to shoot a 22" barreled 300 mag, I'd just as soon shoot a 308".
With most guns there is very little to be gained from barrels longer than 24". With all guns there isn't enough to worry about with only 2" difference. If you are deciding between a gun, even a magnum round, with a 24" vs 22" barrel there are more important things to influence your decision.
Here is another good read. The author tested multiple rifles by chronographing, cutting and chronographing again at different barrel lengths. Here are his results.
http://www.longrangehunting.com/foru...elocity-19346/