I personally find that adding 1 cup of 10:1 Mg salts to 15gal of water change water made with IO that I change weekly keeps my tank around 1400ppm. The reef chemistry calculator
http://reef.diesyst.com/chemcalc/chemcalc.html works relatively well for calculating a starting point. This article might help too.
Do-It-Yourself Magnesium Supplements
for the Reef Aquarium
http://reefkeeping.com/issues/2006-07/rhf/