The typical figure is 0.36 qt per lb of grist. That's for the material that remains once the starches has been dissolved.
It's what's built into most calculators, including this one: http://www.rackers.org/calcs.shtml
Thanks. I'm guessing now that beersmith does this when calculating batch sparge volumes. Although in the past it has occasionally told me to use a pre-boil volume that was greater than my kettle!