"Impossible to rely upon with the varying concentrations of calcium and magnesium present in municipal and city tap water."
That's very true, but also includes any mineral elements that could be in the water supply. For instance, in my area there would be a lot of extra iron that I know for sure. That's also a water quality issue, not a
nutrient balance problem. That's one reason why it's important to use good quality water. Nutrient manufactures can't guess what may be in your water supply in order to be able to adjust for it, and they don't even try. They base their formulas and recommendations for its use on a good quality filtered water like RO water. Or even a pure distilled water, but manufactures don't expect you to buy a bunch of distilled water, so they generally base their
nutrients on RO water. It's not their fault if you use something else. Their are company's that will manufacture nutrients based on your specific water supply though.
I don't doubt that there may be times when a manufactures batch of nutrients may not be up to their standards, yet somehow still make it to market. But that's really about their quality control process, and I'm sure would be quite rare. Even so, there are no EC/TDS/PPM meters that will be able to tell you what the individual levels of each mineral element actually is. They can only tell you the total amount of all the mineral elements combined. The only way to know individual mineral levels is to make the nutrients yourself so you know at what ratios you made them, or have them lab tested to find out. And unless you know what the levels of the mineral elements are to begin with, it's just trial and error to try to change them. And again, unless you make your own, you can only raise the levels by adding to them, because you cant take them out of a manufactured nutrient.