.....data from a 10" 'test barrel" into your revolver's barrel length.
Use a Chronograph.
Doing it mathematically is about as feasible as converting copper crushing units to psi.
PS: Alliant web data says 300-MP can get 1450 fps out of a '10" barrel" with a load of 22.3 grains.
Their 2400 powder can get 1409 fps with a charge of 17.5 grains.