You have BeerSmith II, correct? You need to set up a custom equipment profile for your specific equipment, and like BenB said, BeerSmith will then calculate the correct total water volume for you.
So, say you lose 0.7 gallons/10lbs of grain to absorbtion, then your particular pot loses 1.5 gallons/hr. to boiloff, and you lose about 0.5 gallons to trub. BeerSmith will then calculate your total water volumes, including sparge volumes if you select that as your brew method. If this were a full-volume BIAB, like I do, BeerSmith would give me a starting water volume for a 5.0 gallon batch with a 10lb grain bill of 5.0 gallons (final volume) + 0.7 gallon (grain absorbtion) + 1.5 gallons (60 minute boil boiloff) + 0.5 (trub) = 7.7 gallon total starting water volume to end up with 5.0 gallons.
But all of these variables are specific to your equipment and process, so you really need to dial in these variables for your brewday and enter them as a custom equipment profile in BeerSmith.
