Hey Tom,
I am a BIAB brewer and will agree with what everyone has said so far. My worst efficiency was 55%, my best was 78%. There are three things that really affect my efficiency. Crush, Time and draining.
For the crush, my mill tends to loosen up over time and when it does, my efficiency drops. If I have a bad efficiency day, I'll tighten up for the next brew day and it will usually improve it up to 10%.
Mash time- I know there is conflicting science on this, but I personally find that mashing for 90 mins vs 60 will help my efficiency.
Draining- it's hard to completely rinse the sugars out of the grains using BIAB. I have modified my process from the typical BIAB to include a rinse/sparge stage. I hang my bag from the rafters of my garage, over the pot. I have a ratchet pulley to help lift and keep it suspended. When designing my recipe in beersmith, I will put 3-4 gallons (for a 10g batch) in the "kettle top up" field. This makes the mash a bit thicker. Once the mash is over I will hoist the grain bag, then pour over the 3-4 gallons on top of the grain bag. This gets more sugars into the wort, and also allows you to make a bigger beer with your pot!
As the boys have said- it's really not the number that matters. At our scale it's not costing you much (other than pride
). My biggest issue is keeping it consistent. I don't mind getting 65% when I planned the recipe for that.