Typically with Brew in a Bag, you would start with full volume or nearly that so your mash would be thin. You don't have to worry much about sparging. I've done 3 batches, twice without sparge but squeezing the bag and one with sparging and squeezing and I got nearly the same efficiency either way. Since you have all the mash in the bag all you have to do is get most of the wort out at the end.