A few days ago I brewed a 5-gallon BIAB batch, used 7.75 gallons of water, based off a 60 min 1.25/Hour boil off rate, .25 trub, and a .045 per gallon grain absorption rate. I ended up with a little over 5.5 gallons in the Carboy; planned OG was 1.064, I had 30.7 PPG with an efficiency of 83%...