But what are your allowed ranges? Everyone's 'correct' range is different. Alk could vary from 7.6-11 depending on who you ask.
Typically, my Alk is between 7 & 8 dKh, my Ca is between 400-500 ppm and my Mg is around 1300 ppm.
I am not taking special action as long as the ranges are:
Alk: 7-11 dKh
Ca: 350-500 ppm
Mg: 1200+
IMO this article is really useful to explain Alk, Ca, Mg and other params:
http://reefkeeping.com/issues/2004-05/rhf/
From what I know, the normal Alk could be from 7 dKh to as high as 11-12 dKh. The "ideal" value depends on the amount of borate alkalinity in the tank. And some folks keep it higher intentionally considering that it helps corals growth, although a higher Alk level usually leads to more precipitation, so more maintenance/cleanup of equipment.
Most tests measure the total Alk, and most reef tanks have a somehow elevated borate Alk as compared with natural seawater, therefore the carbotane Alk which is the difference between total Alk and borate Alk could be low if the total Alk is only around 7-8 dKh. The corals need the carbonate Alk, the borate is usually present in artificial salt mixes in higher amounts that natural seawater in order to stabilize Ca & pH. IMO, it is important to know the borate Alk in your tank in order to determine the ideal total Alk value. I usually measure borate Alk twice a year, I have a Seachem test kit that can measure borate Alk beside total Alk.
Some folks try to keep the Ca higher than natural sea water, again considering that it helps increase the coral growth. The article I pointed to above, says that it doesn't and recommends a range close to the natural seawater.
As for Mg, there are opinions that it helps control algae blooms if its levels are higher. I do not know if it's true or not and what levels it needs to be to work against algae. My goal is not to have it lower than 1200 ppm because it is needed to prevent the Ca from precipitating, aka to keep a good level of Ca in the water.
Hope this helps.