It works for me to use my own simple formula. i do 11-gal BIAB batches in a 20-gal kettle... I started wtih using 3/40 extra gallons per pound of grain, plus 1.5 gal for boil loss, and an extra 1/4 gal when there's a big hop bill. So, if I had a 25 pound grain bill, I'd use 11 gal plus 75/40...