Just went back and looked at the original post here. I learned
1. You have to read the post carefully
2. The report tells you what the magnesium level is:
Originally Posted by Lodovico
Calcium- 88 ppm
Hardness as CaCO3- 243
What's the magnesium??
Calcium at 88 mg/L corresponds to calcium hardness of 50*88/20 = 220 ppm as CaCO3. Total hardness is 243 which means that the magnesium hardness is 243 - 220 = 23 less the hardness from iron, strontium, copper .... That makes the magnesium 12.15*23/50 = 5.6 mg/L. This is subject to error if there are other polyvalent cations in the water but should be a much more accurate estimate than the electroneutrality one I gave earlier.