If you have true black powder then you can weigh the amounts your dippers hold. But this applies ONLY to proper black powder. If you are using loose Pyrodex or 777 powder then all bets are off and you need to wait for a muzzle loading powder measure.
If you look around there is no such volume measurement as a "grain". The only world standard measured in "grains" is a weight. Instead history and convenience has given the muzzle loading community this measurement system based on a volume measure that is supposed to drop that same weight of black powder. After all, who wanted to carry a scale around in the woods or on a battle field?
And yes, the granulation size does matter. But not by as much as you think so it's still in the safe range of use.
This whole practice is based on the idea that black powder has a pretty narrow range of density variation. So a 100 grain weight of one brand and granulation is pretty close in volume to any other brand or granulation.
Here again keep in mind that the standard for this "trick of convenience" we use as muzzle loaders is based on proper black powder. Pyrodex and 777 have a different density so the same VOLUME has a different weight. So you can't weigh charges of Pyrodex or 777 unless you base it on the a test of the volume measure using black powder first.
I don't know how much of a charge you want to use but I can tell you from getting ready for a recent day of BP shooting that a .30-06 casing holds 62 grains of black powder. I used that as a measure when I could not find my adjustable measure due to misplacing it. The weight/volume is based on the case when filled, tapped lightly to settle it then crowned off to just overflowing so it's a consistent "head" with 3f black powder.
You're going to find that only the biggest and fattest of the Lee dippers, if that is what you're using that is measured in CC's, hold enough loose charge for muzzle loading. And pouring those into the muzzle is a messy business. That's why I chose to use a bottle neck rifle case for a measure. In my case I was aiming at 62 grains and the stock casing gave me that. But if it had held too much I was going to shorten it a little. Depending on what amount of charge you need you might look at a rifle casing as a measure as well.
Here is a link to a page that has a list of common cartridges and their capacity for a weight of water in grains. Water is slightly heavier than black powder. Also there will be a slight variation between brands of casings due to variations in the wall and head thickness. But this will get you started if you like the idea of a rifle case for a powder measure.
http://kwk.us/cases.html
Two reasons I like using a bottleneck rifle casing as a measure is that first the narrow mouth fits into the muzzle. So little or no spilled powder. Second is that the smaller mouth and the amount of powder it takes to give a free poured slightly overflowing crown is going to vary less from pour to pour because of the smaller size mouth.
