On the question of double milling (or milling grain fine) with BIAB, I've done both standard milling and double milling in my BIAB experience. When I mail order my supplies I can't get them to double mill even if I pay extra so I have to add a couple of pounds extra base malt to hit their expected gravity in their kits and recipes. When I go to my LHBS, I double mill since customers do their own milling there. It's been difficult to scientifically determine if there's a difference because other variables are at play - for example gravity strength of the beer you're brewing - which reduces efficiency. Plus I've not kept good notes on brews I've double milled. Intuitively, if you crack the grain more you should see better efficiency but I suspect there's a limit. Cracking to the point of 100% flour - while not a problem with a BIAB no sparge approach - might have other drawbacks as mentioned like the particulate matter coming thru the bag into the boil. Testing this out would be interesting. Sounds like a Brulosophy exbeeriment !