Refractometers were designed to measure the concentration of simple sugar solutions. They are inherently inaccurate when measuring wort, and become even more inaccurate once fermentation has begun. I love my refractometer for readings during the mash, but if you want to use it more generally you must understand its limitations first. If you want to estimate post-fermentation gravities, you need to use one of the correction formulas out there.
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