If your mag is chronically low, your sps be unhappy way before you experience issues with precipitation. I've had birdsnest and undatas rtn with mag less than 1000 before I understood the importance of dosing mag too. Don't forget magnesium is the third most abundant element in ocean water and plays an important role in the interaction of alkalinity and calcium.
I don't put much stock in the 1ml/gal dosing guideline. I'm dosing almost 400mls a day of ca in my 150g water system. I have lots of sps, including a 18" monti cap. If I were starting with what I have now, I might opt for a ca reactor, but I started with no corals and worked my way up, so dosing made lots of sense back then.
Whether dosing or using a reactor, both methods of adding supplements require consistent water testing to ensure appropriate concentrations. The beauty of dosing is the simplicity. If my water tests lower than 400 ca, or my alk below 9, or mag below 1300, I add minutes to the time the dosing pump runs. Because my corals are constantly growing, the amount of supplement required is continually increasing over time. And I can alter the dosing on any of those three items independently of the others.
I also am not concerned with dosing the exact same quantity of alk and mag as ca. I dose more ca than alk and way more of those than mag. In approximate ratios, my tank requires roughly 5x ca, 4x alk and 1x mag to keep my water "in the zone".
