The volume of one grain of fffg blackpowder is 0.0628 cc
The volume of one grain of pyrodex p is 0.0823 cc
To find the weight of pyrodex p that is equal in volume to 12 grains of fffg blackpowder; 12 x 0.763 = 9.156 grains.
I hope that makes sense!
Daz