Flintlock pan powder

Before I do anything stupid, is it possible to "grind" down 3f or 2f and approximate 4f?
Would there be any advantage?

I put some 3F or even 2F in a small plastic bottle with three or four lead balls. After a bit of shaking and/or trips to the range, I get something close to 4F. I think I read this on this forum a long time ago.

The only reason I want powder finer than 3F for priming is that I use a primer flask with the spring valve. The finer powder feeds through the valve more consistently. 2F actually works fine, just more trouble to get into the pan.