I use anywhere from 1.5 - 2qts/lb. Usually try to balance the strike volume with the sparge volume.
And I may be mistaken, but just because you have a gallon under your FB doesn't make that deadspace. Calculate deadspace by what's leftover after you've completely drained your MLT. For example, I have about 1 gallon under my FB too, but my deadspace is only about 8oz. That's what I can't get up with my pick up tube.