You must measure real black powder by volume, do not weigh it. The other great thing about Black powder is that using a little too much isn't going to blow a gun up, unlike smokeless.
WRONG!! A grain is a measure of weight, not volume. One grain Avoirdupois scale is 1/7000 of a pound and is the unit of weight used on powder scales. However, it is common to create a volume measure that holds a know weight of powder then use that volume measure in lieu of weighing each charge. A fraction of a grain is not considered as critical in measuring black powder or BP substitutes as when measuring out smokeless powder that can create much higher pressures. So weigh out 10 grains of your FFFg then create a measure that just holds that same volume and carry on.