PPM measurement is kind of tricky… Like the HM Digital TD EZ for example. The measurements are calibrated to NaCl. I called them up and confirmed that for other mineral content, this is only an approximation.
It is actually, quite the rough approximation. However, if you know the relative conductivity for the type of minerals you are mixing relative to NaCl EC value, then you can reverse back into a true ppm vs. assuming all particles are NaCl.
I bought a Hanna instrument for measuring magnesium and calcium content precisely but haven’t got around to it yet.