Blackpowder, unlike smokeless is very forgiving...It's just not necessary...Try this, measure 2 loads 2 grains less than your gun's favorite, 2 dead on and 2 loads 2 grains over...Shoot 3 targets with each loading in one target and compare...

Another way to put it, blackpowder doesn't pack the punch that smokeless does, so a few grains either way won't make much difference in velocity...