Anyone have a calculation for how much water a pound of grain sucks up and holds onto? I want 7 gallons of wort, so with two runoffs I want to target 3.5 gallons per runoff. For the first runoff, I know the amount of water that I used in my mash, say 3 gallons. I need to know how much of that initial 3 gallons will be retained by the grain, so that I can determine how much mash out/sparge water to add so that when I do my first runoff, I'll yeild 3.5 gallons.

I'd guess that the grain holds on to about one third of the mash water, so my initial 3 gallons is reduced to 2 runoff gallons and I need to add 1.5 gallons for the first runoff. How does this estimate sound?

Then for my second runoff, I simply need to add 3.5 gallons to the grain, because the grain has already sucked up what it is going to keep, and what I put in should equal what I get out.