The ratio of Magnesium sulfate to Magnesium chloride depends if you are using a two part solution, kalk or reactor for Ca and Alk. I use Kalk and mix the two Magnesiums based upon BRS directions on the Magnesium chloride container. Magnesium sulfate is Epsom Salt so I get that at the store.
Mine was quite low in that I hadn't checked or supplemented it in months, and I am slowly getting back up. I do see some improvement in my coral and I am seeing new coralline algae growing.