Don't use a salt mix. Use table salt (iodized or not doesn't matter).
The recipes are different for refractometers, conductivity devices, and hydrometers.
Weighing is better than volume measurement of the salt, if you can do it.
For a refractometer:
Use 3.65 weight percent sodium chloride solution to match 35 ppt (sg = 1.0264). It can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.
For a rougher measurement in the absence of an accurate water volume or weight measurement:
1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.
If you have a 100 mL graduated cylinder, use exactly 3.65 grams of salt and 96.35 mL of fresh water. It is tricky to volume measure such a small amount of salt, but that is about 0.59 teaspoons of Mortons.
