there are a lot of different opinions on here. I have my own grain mill and my first AG was about 78%. I keep mine at the factory settings. It is supposed to be set up since they worked with the homebrew shop that I purchased it from. I have a barley crusher.
Too fine and you could get a stuck sparge. too course and you do not get the extraction that you want. Try and move it in a little and see if it improves.
As far as efficiency goes, I dont think you can go to fine. As mentioned though, if you end up making flour, you are going to have one big ball of goo that is not going to lauter.
Mine is at .037 now and I use a round cooler with a stainless false bottom and get no stuck sparges. I get 75% to 80% efficiency and that is what most brewers get that are mashing and sparging correctly.
If you are getting poor efficiency it could be due to not hitting your mash Temps or using to much water when mashing also PH could be a factor in the water your using.
Im using the 10 gallon rubbermaid, with the braid, the flow from the tun is very fast, so Im nowhere near a stuck sparge, I will try again soon, thanks fir the replies guys.