refractometer

As far as the calibration works. It is more accurate to calibrate to the range that you will be using, the same goes for any testing instrument. Calibrating with RO or Distilled will give you a great 1.000 calibration (0 ppt), but there can be issues with the instrument (slope miscalibration) that can lead to it being slightly off by the time you get up to 1.026 (35 ppt). There is a good article in reefkeeping magazine, I'll see if I can find it.

EDIT: Here it is, I hope it's ok that I link to another reef site. http://reefkeeping.com/issues/2006-12/rhf/index.php

 

This is what I use.... Conductivity Calibration Solution - 53.0 mS: Premium Aquatics

 

I believe that's what I'm using now but I just found this at BRS and it looks much nicer just because it comes with the dropper cap so you don't need to use a pipette/dropper.
Aqua Craft Refractometer Calibration Fluid - English