The 250 grain FTX is .750 long and well with in the capability of a 1-32 twist. You are correct in believing that the slower the twist the shorter bullet that matches it.
I ran a few numbers to give you some ideal length bullets:
.452 ideal length .95
.458 " " .97
There are a couple of things to remember about this:
It is much harder to over stabilize a bullet that it is to under stabilize it, this means that to long a bullet will be harder to get to shoot that one that is a bit short.
Also remember that changing a bullets velocity changes the speed of its spin all of these are calculated at 1800 FPS some times when a bullet is on the fringe a faster or slower velocity can help. A LOT of the bullets we normally shoot will do fine in a 1-32 twist. You may have to push the 300 grain bullets at least 1800 FPS to get them to shoot well.