An update; I finally got around to getting some of the pinpoint conductivity calibration fluid. Premium aquatics sells this as "salinity probe calibration fluid" and it's a known standard for correct refraction at 35PPT. So, if you buy this stuff ($3.45) you can calibrate your refractometer so that it is very accurate at typical reef aquarium salinity. Calibrating a refractometer to read zero with R.O. water doesn't ensure that it will be accurate at 35PPT, especially measuring ASW as opposed to NaCl dissolved in water.

Anyhow, after all the reading I did about this, I finally tested my refractometer, which is one of the sybon one. It was right on the money, which really surprised me.