I don't know, and I think it would be affected by how coarse/fine your milled grains are, but here is how I would figure it out... mill your grains (of known weight), dump them in a (empty) fermentation bucket (with gallon marks), and see how many gallons of space the milled grains take up. Divide that volume by the weight to get volume per pound. This will tell you your worst case displacement, which is what you will very temporarily have when you first put your grain bag and water together, before the grains have a chance to absorb any water. (Actually it will be worse than worst case (i.e. it will be conservative), because there's no way you can get all the grains submerged without absorption starting.)
