If you ever hear people talk about TDS measurements, you'll notice that they often indicate (or should indicate) whether the measurement was taken at 0.5 or 0.7 conversions. Those numbers are important because it ensures that you're comparing apples to apples when discussing TDS.

There's nothing wrong with the calibration solution they hydro store sold you. It's a standard TDS solution that a lot of us use. The problem is you need to figure out how to set your TDS meter so it accurately reads the calibration solution. To do that, you need to be sure that your meter is set to either 0.5 or 0.7 (your meter can do either), and then *manually* calibrate it to match the number on the bottle - 1500 for 0.5, or 2190 for 0.7.