Not to get technical but your refractometer measures salinity in ppt (parts per thousand) so even if your RO water was way out at 35ppm (parts per million) you'd still only be out by 0.035 ppt on your refractometer. Accuracy on a scale measured by your eye is equal to +/- half a mark which in the case of all the refractometers I've seen is 1 ppt, so you're refractometer even if calibrated perfectly is technically only accurate by +/- 0.5ppt. I wouldn't worry about that extra 0.035ppm, you couldn't even tell the difference when you calibrate it. :rolleyes:

Some salinity tips from Randy Holmes-Farley: http://reefkeeping.com/issues/2006-12/rhf/index.php

Fwiw, my hydrometers are out by 0.007 and 0.008.

I use DI water, sometimes even tap water and have seen little to no difference when calibrating my refractometer at the 0 level and that's with 210 TDS tap water. It is of course possible that the 'slope' of your refractometer could be off making the 0 cal inaccurate at the salinity range you like to keep your system at. It most likely boils down to each individual refractometer. All I know is my critters are happy & I've never used calibration fluid to dial in my refractometer. I didn't pay enough for the thing to warrant using lab grade calibration fluid on it. I reckon as soon as you open the calibration fluid bottle or whatever it comes in, it's no longer useful for future use. If I was going to get that precise in calibrating my instrument, I wouldn't use the fluid more than once. I'm more concerned about ensuring my water change water is as close as possible to my system water since I tend to do fewer, larger water changes. This is where I value my refractometer, whether it's off .00? points or not, as long as both readings, make up water & system water are within .002 of each other, I'm good.