don't buy a cheep refrctometer. there are too many things that can make them inaccurate, although precise,
firstly, you have to understand that refractometers do not measure salinity, they measure refractive index, there are many things that affect refractive index, most notably magnesium, which is a major component of NSW, in other words, you can have a refractive index that would suggest .35ppt, however if your MG is off, and it quite often is, that will not be your salinity.
also, always use .35ppt calibration solution, I do not care what the instructions say, don't trust zeroing with RO/DI for calibration... to do that you are assuming the correct slope in your prism...and that's a very big assumption
always buy a refractometer with temperature compensation and always wait 1 minute before taking a reading, temperature differentials play havoc with refractive indexes