Question

who I can measure Mg whitout Ca?

Answer 1

Explanation:

I am quite aware of the definition of hardness and the many units it can be expressed in.

In practice the hardness of natural freshwater comes from Ca and Mg unless it is anaerobic. If it it anaerobic Fe and Mn can also contribute.

I disagree with your labeling of the hardness parameter as "indirect" and "approximate" as the parameter is defined by the analytic method (titration by EDTA) and not an equation. Hence the results of the titration is the true value. Also it is quite repeatable and exact if it is performed by a skilled analyst.