The link for homemade 2 part additives:
http://reefkeeping.com/issues/2006-02/rhf/index.php

Once you get your Ca and Alk levels where you want them, if you dose both parts you really only need to check Alk frequently. The Ca levels will follow the Alk levels, although the Alk will drop and rise much more quickly. This is because there is much less carbonate than Ca in seawater.

Regarding Mg supplementation, ideally over the long run you'd use a combination of Epsom salts, which is magnesium sulfate, and magnesium chloride, which is cheaply available as Magflake. The ratio is discussed in the 2 part recipe. The reason is that you ideally want to keep chloride and sulfate ions in NSW proportions.

If you use alot of 2 part, whether it's homemade or B-ionic, you're going to eventually increase the salinity of your tank. This is because you're adding Sodium Carbonate (or Bicarbonate) and Calcium Chloride. As the Carbonate and Calcium get used up, what's left? Sodium Chloride, or basic salt. Don't worry too much about it, as there are 19000 PPM Chloride and only 400 PPM Calcium, so it takes ALOT of Calcium Chloride to upset the Chloride ratio by any substantial amount. A similar situation exists with Sodium and Carbonate. If you do regular water changes, you'll keep a reasonable ionic balance. Randy Holmes-Farley does a good job of explaining exactly how this works at the end of the recipe.