I would just screen the FFFg or FFg and save the dust and use it for priming. Just find a screen with a mesh that will pass 4f and smaller.
There was an article in Muzzle Blasts about serious target shooters rescreening their black powder for more consistant shot to shot performance. It could be that the difference between normal grades of BP and the premium grades is more careful screening. When I pour Schuetzen into a funnel to fill a powder flask, I notice a lot of dust. With Swiss and the premium priced Goex Cartridge, there seems to be little or no dust.