Creating a standard solution yourself, which matches your sample stream more accurately, is a way to get a more accurate calibration. A proportional mixture of salts based on your sample stream can be added to distilled water, where 1mg of your salt mixture 1l of distilled water will yield a 1ppm TDS solution. Try to produce a standard that is close to your expected sample in concentration, or If you expect a great deal of variation make a solution that is in the upper third of the sensor range.