I have a dd h2ocean refractometer atc. Instructions say to calibrate it to zero using pure water. When I do this and measure it against 35ppt solution it is off. Which should I use to calibrate? Rodi water or 35ppt solution. My refractometer says it should be calibrated with solution that is 35ppt at 20degrees but my solution is 35ppt at 25 degrees. Does this matter?