Use this calculator to fill out the “BIAB Grain Absorb” preferences field in Beersmith. The field takes a value of fluid ounces lost per oz of grain. Based on the way I squeeze the bag my loss compared to the default for a 10lb grain bill was leaving me with an extra 1/4 gallon of wort.
Make note of your starting BIAB volume and your pre boil volume to determine the gallons lost. Enter that along with lbs of grain used in the calculator below to get the fluid ounces lost per ounce of grain.
Use the value from Fl oz Lost per oz of Grain to update your preferences and better dial in your system.