I vary my ratio according to the grain bill to allow me enough sparge water,but not too much. With 15 lbs of grain, you don't get much sparge water at 1.5 qts/lb.
I use a very scientific approach to this...what ever ratio gets me closer to half unit measurements in my mash and sparge quantities. (Meaning I'd rather measure out 3.5 gallons and 4.5 gallons then 3.3253 gallons and 4.653432 gallons).
Reading over Brewing Better Beer by Gordon Strong and it mentions that thinner mashes may cause too much dilution and enzymes may not convert as much, as good, as fast... So I would stay away from getting the mash too thin. On a typical brew day he suggests 1.5qts water per lb of grain. I do like one of the above mentioned posts that if you have a NON-heated mash-tun going a little lower offers the ability to adjust temp if need be by adding more water without running the risk of getting a thin mash. I think I will give that a test run, starting with a 1.25qt ration then adding more as needed for temperature correction.
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