Might the length of sparge have to do with the amount of sweetness remaining after the desired volume has been drawn off? For me that is sometimes a problem. I get decent efficiency, but with some 10lb grain bills I note a gravity of 1.015 or a bit higher in the last runnings. The sugar is converted and available, but it doesn't seem to all rinse out with the 6.5 gallons of water I start the boil with. (Mash with 1.5 qts per lb, split the batch sparge water, do the stirring and waiting, temps are fine.) I know I could run more sparge water through and boil it off, but I suspect there's a way to get more good stuff without.