If it helps ,
I mill the Kno3 and sulphur until super fine , i then screen all the ingredients together through a 60 mesh screen about four times and use .
I used to do this and in fact I wasn't too fussy about the charcoal. It came "air float" but also contained some much larger particles. They worked like a charm. It was when I started ricing the same comp that things got a bit variable.
I quite like the short mill for my tests as it really ensures the BP is reliably well integrated. It's no bother throwing a batch in the mill for an hour. Once I have a number of reliable tests under my belt I may change things a little, like this.