You're over thinking it. What I do is calculate the water needed based on the grin bill, then add some to account for the space under the false bottom. Then I use that number to calculate my strike water temp. You don't have to worry about it "never touching" your grain, you might have some dilute wort under there, but a little recirculation (which you'll do anyway for at least a little while) will redistribute that.
Water needed is on the left side, under tools.