Perhaps this will help muddy the water - or help. When I work up loads for a rifle, I use an adjustable measure, marking on the target the setting of the measure. When I find a load that shoots, I then make a measure that throws THAT amount, weigh the powder charge & record it, so I can make another measure to throw that amount if I need to.
I have an old adjustable measure that throws almost exactly what the markings show, when using 3f GOEX, just lucky, I guess.
I found in testing measures for accuracy, that the 5/16" ID measures made from brass tubing throw much more consistent charges than the 3/8" ID measures throw. As well, the 3/8" ID measures are more consistent than the 71/6" ID measures, etc. The worse, were the horn measures with tapered 'cavities' and fairly large top openings. Yes - they look great - but stink as an accurate measure.
The measure I use mostly for my .32, has a 5/16" ID. It throws weights that vary barely 1/10's gr. each side of the target 35gr., throw to throw. My measure for the .40, is under 3/8" and it does not vary more than 1/2gr. each side of the target measure. The method and care used to pour the powder also has a large effect on the accuracy or consistency of the charges thrown.
Black powder is, of course, not as particular as smokeless powder in these regards and certainly not as dangerous to have large variations, however the close your charges are, load to load, the better will be your consistency, overall.