If ordering from AHB it seems that they crush assuming you will obtain about 68% efficency with their standard crush by my calculations. Since purchasing a mill I am getting 82 to 83 percent efficiency through batch sparging. Whereas I was about 70% pre mill. So to make a long story shorter, I don't know that you need a double crush to hit OG with the generous grain bill in their kits. If you are formulating your own recipes I apologize as this post is irrevelant.
Edit-sorry or irrelevant, missed BIAB.
Would a cheap corona mill be good for BIAB?