Your TDS meter read's the concentration of ALL conductive ion species in water (sodium, calcium, chloride, nitrates, sulfates, etc, etc, etc). If it is not accurately calibrated to known chloride standards with a calibration factor determined for your pool water, you will not get accurate reading. That means that you must know the exact concentration of your standards to ensure the meters linearity AND you must know the concentration of chloride ions in your pool water in order to establish a calibration factor for the meter.
The only accurate method of measuring salt content is to use a Taylor K-1766 test kit. It is a chemical measurement of the chloride ion (Cl-) concentration in the water which is the only thing that matters for an SWG. Anything other type of "salt" measurement is a proxy-method which is subject to huge variations depending on what else is in the water, temperature, etc.
Your water can easily have upwards of 1000ppm of salt (chloride) in it already from acid (hydrogen chloride) or liquid chlorine/bleach (hypochlorite degrades to chloride ion).
