Camel (I was more of a no-filter guy, back when I smoked),
Sorry for the really stupid math error.
...I guess I should stick to typing while completely sober!
In that case, you're not getting the water out that you put in.
Losing 10% of your sparge will cost you 4% efficiency, alone.
I got 50% efficiency from one homebrew shop's mill, went to another and got 65-70%, got my own mill and got 75%, optimized my crush and now I average 80% efficiency (with a couple around 90% with a double crush).
I shouldn't have given such a terse answer, I guess.
First, what I meant was that adding grist to water is a perfectly acceptable way to do it, not the only way. I think it can help in avoiding doughballs, but mostly I add the grist to the water because it allows me to double check the strike temperature. Having the water in the tun allows me to make sure that the predicted amount of heat was lost to the tun and avoid surprises such as those caused by an overly cold day or tun which results in a missed mash temp. I live near Boston and we have some pretty wild temperature swings in just a few hours, so room temp doesn't mean much. I weigh out all my grain and grind it right into the waiting strike water.
Second, a lot of people stir between mash in and mash out, but it doesn't seem to make much difference in efficiency, according to Denny Conn. I've never done the side by side experiment, but the top 5 efficiencies I've had were after stirring in, stirring just prior to first runnings, and stirring prior to 2nd runnings, only.
Third, I do something similar, I just use a plastic Indian takeout food cover that happens to float. As I don't fly sparge, I don't worry about the top of the grain bed much, but I hope it helps cut down on hot side aeration.