Mine was less than 20 on Homebrew Finds, it is always spot on. If I had two, one or both would always be wrong. Segal's Law
I'm not building Nuclear bombs, so if it calibrates well at 0 and then in a 5 or 10% solution then I'm happy. The other night I did a full test on every sample between my hydrometer and the refractometer, per the conversion table they were within 1%, good enough for me.