For me it depends on the OG of the beer. I mash as thick as 1 qt/lb for high gravity brews to give me an adequate volume of sparge water, as thin as 1.5 qts/lb for session beers, and ~1.25 qts/lb for med gravity beers. I typically double batch sparge, and based on Kaisers experiments, maximum efficiency comes when the run-off volumes are close to equal, so that's what I roughly aim for. Mash thickness does effect ph to some degree, so that should be kept in mind if you're messing with the water profile.
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