Interesting. I'm still learning my BM20, and I was going to start opening up my mill gap, but now I might close it more. It's crazy how different results are across users...
Crankandstein 3GT at 0.045 gets me ~65% mash efficiency with 0.9mm filter.
IMO, the accessibility of starch is one of the key factor of mash efficiency. In malt analysis sheet or specification, DBFG(fine grind) is always greater than DBCG(coarse grind). Moreover, the specification used in Europe specifies in mashing time which is usually 10 or 15 minutes, which suggests that converting finishes in 15 minutes if the malt is milled as fine as it is in the test procedure.
I set my mill gap to finest when brewing BIAB. When I got my BM20, I didn't change the gap. I am getting satisfying efficiency for recipes of less than 12lbs grains.