I just did a 5g batch AG with 20lb of grain, largest I have done by a long shot. I did my standard 1.25qt/lb for mashing, but ended up with only 2 gallons to do a double sparge. Needless to say, my efficiency was crap (50%, normally I get mid 70's) as I figure I didn't wash the grain with enough water during the sparge. Mash temp was 152F for 60min, and sparging got the grain bed up to 170.
So, with larger grain bills, is it wise to reduce the ratio to 1qt/lb or less so I have enough water to sparge with?
When I figured out what I had left for sparging I knew I had shot myself in the foot, but decided to go with lower efficiency than a smaller batch. I could have collected more, but I would have overfilled my pot and I didn't have the time that night to boil off an extra gallon. Ah well, lesson learned.