I am confused about what you are trying to accomplish.
Start with the premise that blackpowder is always loaded by volumes, not actual mass or weight. Blackpowder is measured with scoops, brass nozzles on flasks, etc. So that means "an equivalent charge" of a different sized powder would simply be the amount of that powder that would fill the measure. Take a black powder cartridge for example: Let's say a .38 Special brass case contains 1 cc (volume) of 2F powder under a particular bullet, and that amount provides sufficient volume to follow the rule about not allowing an air gap between powder and the base of the bullet. The same volume of 3F powder will have a bit more mass, due to less gaps between those smaller powder grains, but could still be called "an equivalent charge" to the 2F powder because it satisfies the golden rule about eliminating the air gap between powder and bullet.
If you are trying to produce the same muzzle velocity with 3F that you got with the 2F powder, then you would obviously need a smaller volume of 3F in the case, and be faced with using some kind of filler between the powder and the bullet. If this is what you are trying to accomplish, then weigh the 2F powder charge precisely on a scale, then use the same actual weight of 3F to get the same mass of powder. (Assuming here that the same actual "mass/weight" of powder will generate the same pressure curve during burning.)
This second case seems like a lot of bother to me, so that's why I asked what you are trying to accomplish, and perhaps what firearm you will be using? One of the joys of black powder is being able to throw volumes rather than measuring actual weights. Another joy is not having to worry about 1/10 grain differences in powder charges!
So you probably already knew all of this, but I'm just trying to explain why I'm confused!