What happens is motion and vibration settles the powder, changing its bulk density in the metering cavity. I would not count on better than ±0.2 grains with Bullseye, though if you get your tapping and feel down, it can do better. Flakes don't cut, per se, in the Uniflow, but if you get them too densly packed they can offer some noticeable increase in resistance to rotation of the cylinder.
Midway sells a baffle for the Uniflow, but you can make your own out of a beer can or even cardboard. Just look at the design to get the idea. You fill the hopper to above the baffle more most consistent effect of gravity on the bulk density by packing. Keeping powder at a fairly constant level by frequent refreshing of the supply also helps limit error, but do throw a few that you return to the hopper right after replenishing to help the powder bulk density settle.
There is more baffle information in this thread