Hi Andrew, and welcome to the forum. There is a ton of different info out on BIAB throughout the net; some accurate but lots wrong or misleading. Think you will notice very good info presented on this site.  I don't think you need to double crush your grain... For top efficiency we would recommend ...

My name is Andrew, from Plano, TX.  I have been brewing for 2 year -- 25-30 batches with a traditional mash tun cooler and bazooka screen and batch sparge. I bought a BIAB for my 10 gallon pot on a Black Friday deal and tried it for the first time last week. I double milled my grain and jumped in. I...