Hi,
Well it's better to keep it simple. The first thing is to determine where you live, because you can go online and find the water quality report for the region your water municipality is in.
That sometimes give you a good idea of what may be in your water.
Depending on the size of your tank, I would add a teaspoon or two of the salt at every water change and call it good.
Again, I would not even think about the actual concentration value, because it won't matter. Mg and Ca simply need to be present in the water. There is no real "official" target concentration level.
Too many hobbyists get wrapped around the axle looking for numbers.
Do you take vitamins or nutritional supplements?
How much mass or weight do those vitamins have compared to the amount of meat and potatoes you have for dinner?
A tiny amount right? A couple of pills weighing a few grams or so. Well there is Magnesium and Calcium in those pills and not a whole lot, maybe a few milligrams.
Does it matter if you take only one pill, or two, or even if you miss taking them for a few days? Not really.
Well it's the same for plants. They don't need a whole lot of these metals, Iron, Zinc, Cacium, Magnesium and so on.
This is good news because you can keep it simple and be worry free.
Throw a teaspoon or two in after you replace the water and be done with it.
Cheers,