Here is my thoughts on supplementing.
For alk only. I have always used Seachem Reef builder. It's not too expensive near me. I only add it when something is out of line with alk only.
For Calcium only. I use Kent's turbo calcium. Again only used when something is out of line with my calcium only
For determining how much of each supplement to use I have been using
jdieck's calculator
http://reef.diesyst.com/
For both my Calcium and alk needs I use both a calcium reactor and a neilson (Kalkwasser) reactor. You can use two part additives (hopefully people will chime in who use them) but for me I find that the calcium reactor is a set it and forget it setup. Once it is dialed in I only look at it once a week to make sure that everything is working correctly, the rest of the time, I don't even worry about my alk and calcium levels.
If you need an introduction to calcium reactors in general. Here is a short primer that I put together to help you understand the basics.
http://www.cnidarianreef.com/faq.cfm?FID=37
As to why I use a kalk reactor in addition to the calcium reactor? I like using kalkwasser as it helps me to maintain a higher pH due to the calcium reactors tendency of pulling down the tank pH. Also over the years I have felt that it does help with growth and coloration to a small extent. Meaning that I can generally tell by looking at my tank when my kalk reactor needs to be filled as the corals just look a little bit different. Perhaps some other people can comment on using a kalk and a calcium reactor together. If you have noticed similar things or is it all in my head.
As for magnesium. Once I get the main tank up and running with the correct magnesium levels. I only supplement magnesium in the water used for water changes. Other than that I never add it to the main tank. I do weekly water changes, so perhaps the frequent exchange of fresh water helps to keep the magnesium levels from dropping.
Also, I do check the alk/ca/mag levels of a new batch of salt before I do the first water change with it. From here I adjust the levels of the newly mixed salt to match that of my main tank for the remainder of the salt batch.