At a salinity of 1.025 Natural Seawater NSW has magnesium at 1285ppmand calcium at 400ppm(roughly 3.25 to one).A proper level of magnesium makes it harder for precipitation to occur and thus makes it easier to maintain calcium.
It is also important to note that alkalinity plays a key role in the calcification process. NSW alkalinity is 2.1 to 2.5 meq/l or 6to 7dkh. Without balanced alkalinity calcium will not be used by your corals.
Using kalk water for calcium and alkalinity promotes growth and therefore magnesium uptake and depletion since corals use magnesium in creating their skeletons in addition to calcium and carbonate
So,balance between these 3 keys:magnesium,calcium and alkalinity (carbonate and bicarbonate)is necessary for coral health and growth. A good bench mark at 1.025 salinity is 1350magnesium with 425calcium and 3.25meq/l(9.1dkh)