Most powders in my area get shipped from no telling where, and slammed around during delivery from every dock worker from here to there, so by the time I get it, the granules have been pulverized to the point that they are not the F size labeled.
I started weighing my charges of pyrodex with a powder scale, so I can keep exactly the same load in my rifle. This is what most of the target shooters do, so I figure there must be a good reason for it.
I measure 3 loads by volume and then weigh each load. I then average the three loads, and they come out to 65.7 grains by weight being equal to 100 grains by volume. (RS Pyrodex)
You can not go by this in every instance as some powder will weigh more or less than others. You must get your average from the powder you are shooting, so that it will be right for your rifle.
I then put my charges in vials that I use for speed loaders, and whether I go to the range or I am hunting, I can be assured that each time I load, will be consistent. By having my charges loaded in vials (about 20 vials) I can got to the range without having to worry about measuring between loadings.