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When I first started tinkering with the notion of going all-grain, efficiency came up all the time. However, I'm starting to lean towards your way of thinking that efficiency is good for recipe building, but how do you know what efficiency to use to begin with? I would think it's more important that the end product stays consistent and within it's style guidelines, then making sure your getting the most out of your system.
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I use experience as my ballpark guess when starting a new recipe. If its a big beer I put 65% in there. Otherwise I use 70% and adjust from there. When I rebrew im usually within a couple points of my targets.
Your last sentence I agree with partially. I dont care about most out of my system. If I use a bit more grain so be it. I care about guidelines only if I send a beer to a bjcp comp, which I've done once so far. I care always about fun, enjoyment, learning, a little repeatability, etc.