I use this recipe from Randy Holmes:
3. Using both Epsom salts and MAG flake, dissolve 7¼ cups MAG flake and ¾ cup Epsom salts in one gallon of water, and use that to supplement magnesium in amounts determined using this linked online calculator, with the entry "Randy's Recipes 1 and 2 Versions A and B," and ignore for this purpose what those designations mean. This recipe is preferred, but its advantage over recipe #2 is minimal in most cases.
Which is derived from:
http://reefkeeping.com/issues/2006-07/rhf/