Make sure you take a reading of the final gravity with a hydrometer. The refractometer only works correctly, as is, with non-fermented wort (sugar water).
Check your hydrometer with 100% distilled water, should be 1.000 sg. If anything different you will have to add/subtract from your beer measurements by the same amount the water was off.
PS> there are formulas that can approximate FG from refractometer, but it's better to just use the hydrometer for this purpose. Refractometer is fantastic for everything pre-fermentation.