For me 20" is as short as I'll tolerate and 24" is as long as I'll tolerate. It has more to do with balance and muzzle blast than bullet speed. There is simply no way to predict exactly how much speed you'll lose with a shorter barrel,from another gun. But I've looked at a lot of data. With almost any chambering there is very little difference between 20-24". Going shorter than about 20" often results in large velocity drops, going longer than 24" usually results in almost no gains.
I you were to use one of your existing barrels and cut it shorter it would be easier to predict. Somewhere between 15 fps up to 25 fps for each inch you go shorter would be in line with what others have gotten when they cut a barrel.
But starting with a new barrel is just a wild guess. I've seen 2 different guns, with equal barrel lengths shoot ammo from the same box 130 fps differently. I've seen a 20" barrel shoot faster than another gun with a 22" barrel with ammo from the same box.
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id be dubious about 18 inches because of the slower burning H4350 powder
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This is a common misconception. If the powder you are using is getting the best speeds with with a long barrel, that is the powder you will get the best speeds with from a shorter barrel. Using a faster powder for shorter barrels is a myth.