I do dose mag when needed. I do keep a log and in the beginning I test often to get a feel of how my tank is using the big 3 (mag, cal, alk). After awhile I can start to understand how my tank is using the 3 and I can cut back on the testing somewhat but still test for the three when I visually see something may not be right. Hope this helps.
Use Salifert test kits you will thank yourself later. For hobby grade test kits I think they are the best. Of course there are a few other brands that are good but I use Salifert as my main kits. I have some La Motte kits for cal and alk that I will use to compare my Salifert results with and make sure I am getting the same numbers. Both of them are always right in line with each other. So I am confident my test results are accurate.
This is taken from Randy Holmes Farley's Reefkeeping article.
I suggested to you once before I would read all of his writings.
"Magnesium's primary importance is its interaction with the calcium and alkalinity balance in reef aquaria. Seawater and reef aquarium water are always supersaturated with calcium carbonate. That is, the solution's calcium and carbonate levels exceed the amount that the water can hold at equilibrium. How can that be? Magnesium is a big part of the answer. Whenever calcium carbonate begins to precipitate, magnesium binds to the growing surface of the calcium carbonate crystals. The magnesium effectively clogs the crystals' surface so that they no longer look like calcium carbonate, making them unable to attract more calcium and carbonate, so the precipitation stops. Without the magnesium, the abiotic (nonbiological) precipitation of calcium carbonate would likely increase enough to prohibit the maintenance of calcium and alkalinity at natural levels.
For this reason, I suggest targeting the natural seawater concentration of magnesium: ~1285 ppm. For practical purposes, 1250-1350 ppm is fine, and levels slightly outside that range (1200-1400 ppm) are also likely acceptable. I would not suggest raising magnesium by more than 100 ppm per day, in case the magnesium supplement contains impurities. If you need to raise it by several hundred ppm, spreading the addition over several days will allow you to more accurately reach the target concentration, and might possibly allow the aquarium to handle any impurities that the supplement contains."