I squeeze as much wort out of the bag as possible. After the tun drains from the batch sparge, I sit the bag in a large mesh colander and let gravity have a go for a while. With a bit of prodding and pushing, I get another gallon out of the bag.
Because of this, I adjusted my Beersmith profile to have a BIAB grain absorption (.586) instead of all grain (.96). This nets me just the right amount of water & wort.