The velocity prediction will be fast by quite a bit, due to the gas loss at the cylinder gap, so what I've done to when playing "what if" is shorten the barrel length to get the velocities to match. I.E, My SuperRedhawk is a 7.5" barreled version, but to get velocity predictions to be in the ballpark, I use 6.4" when asked for barrel length. This was established using known, chronographed loads.
I haven't played with any other revolvers, so I don't know how valid that method is, but it is workable with my SRH.
I've not done anything to alter the pressure given, what with the nature of pistol powders. If the pressure is slightly overstated, then I'll just consider that as a built in safety margin. I've also migrated away from powders that only like to operate balls-to-the-walls, having gotten better accuracy from other powders, especially N110 in the .44 Mag.
It's not much, but that's all I have.
