Since 3 years I personally use an electronic TDS meter Hanna HI96302 with excellent results.
From time to time I check his precision by preparing a solution of 3,5 g of salt in a liter of distilled water and checking that the meter shows the expected value, 3500 in my case (sorry for the measure units, We are in Europe) but I never had to tune it up again in this 3 years periods.
The first time I recommend to test a sample of water to see how many other minerals it contains because the TDS (Total Dissolved Solids) measure all the minerals, not only the salt, then I add this value to the wanted salt level, for instance if the fresh water has 50ppm reading, then my target should be 3550.
So far so good .....