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It seems a common theme that refractometers calibrated using purified water are a few thousandths off. I too bought the 53.0mS Calibration fluid and found that my refractometer has been off 3/1000ths for over a year (like you mine was giving a false high). I posted on here and someone suggested that once calibrated, on a monthly basis or so check it against the calibration fluid. No need to get another refractometer, it would most likely give you a false high anyway. Now that yours is calibrated correctly, it is a very accurate measure of specific gravity/salinity.
BTW, you might want to increase your specific gravity the way I did, which was to replace evaporated water with full strength salt water. That way you slowly, incrementally increase you sg over several days.
Ironically, those cheap swing arm hydrometers were actually giving me more accurate readings than my refractometer before I calibrated correctly. lol
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