I check my measures to see how close they come to the stated values. Like the one I've been usuing lately when set on "40" actually does hold forty grains of swiss fffg. When you load a substitute powder from the same forty grain measure setting, the volume is the same as with black powder- it takes up the same amount of space but the weight is considerably lighter.
So, 40 grains of black powder and 40 grains by volume of black powder are the same thing. When I write up the volumetric equivalent of a black powder load using pyrodex, I use the expression 40grains/Volume or 40 gr/vol just to show that while the charge takes up the same amount of space as black powder, it does not actually weigh the same.
It is likely that the slight variations that might take place from one brand of black powder to another or from one lot to the next of the same black powder or even a bit of inconsistency in pouring the powder would have very little effect on the load.