Believe it or not, velocity and barrel time are irrispective of each other. Two bullets with the same muzzle velocity can have different barrel times, depending on the burn rate of the powder and when and where the pressure curve is. I assumed trying to acheive the same mv with my barrel would get me to a node when switching powders and according to quickLoad, the barrel times were quite different even though the velocities were the same. .
Basically, to answer your question more accurately, You would have to know your case capacity, actual bullet length, COAL, powder used and it would still be a S.W.A.G. Using QuickLoad to guestimate the velocities at each barrel time node.