John,
I have no idea what null-B powder is and I don't screen my powder.
What I have done once was put partley empty cans of 1.5,2 and 3F OE in a coffee can and mixed it to use for blowing out .44 basic cases necked down to the .44-90bn to blow the shoulders out and shot at a 200 yard target. After 100 rounds through that target made me to reload those fire formed cases again and I took them as well as a second rifle to a silhouette match and after getting sight settings I was convinced to use that mix of powder loads and shoot the match using it. I finished with a 29 and a 10 Turkey pin when it was over.
That makes me wonder if worrying about fines in the powder is just a waste of time.