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.
Thanks. So 3 cups in a 50g water change is a good starting point? I guess if I'm going to dump stuff in my tank, I should read all the articles, right??