First off, I don't know how you managed to get 11 gallons into a 1/2 gallon Sanke . Second, no, I don't know the formulas for water expansion, but I can't imagine a google search wouldn't have them. I'm lazy and use BTP. But I would say that for a 2hr boil, going from 11gallons to 8 gallons seems like a high, but quite reasonable boil off rate. That's probably what my system would do (not a Sanke). Looking at BTP real quick, it looks like "11gal" water at 60F is "11.46gal" at boiling.
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