I use the BIAB method. My keggle has a side drain valve with a pickup tube going to the bottom. After the boil, I get gobs of fine material coming through my counter-flow chiller into the fermenter. It must be mostly fine crush flour leaking out of the brew bag during the mash (I use hop sacks in the boil).
I bought some #60 screen (250 micron) and did a shake test on my grain to see what is in there. It looks like maybe 5-10% of the crush is fine enough to go through the screen and my brew bag since they are about the same mesh size. The bag drains like a champ when I pull it, so I don't really want to change to muslin or something else.
Question for the group: Is there a disadvantage to screening my grain to remove the flour before mashing? Obviously I have to make up for missing material in the grain bill, but the $3 more is worth it considering the hassle I seem to have with excess trub and chiller clogs at the end of the run.