I realize this is an old thread, but what's the difference b/w calibrating your refractometer with some "known concentration" fluid; as opposed to simply using RO/DI water to calibrate it to zero? The RO/DI water is free, and there's zero unknown. Am I missing something?
Read the first review on the product page and then check over on Reef Central and you'll see why you should use a calibrated solution to calibrate any of your equipment. Lots of discussions on this topic over there.