This kinda gets away from the OP's original ? and into a Mg chemistry deal, but anyway...
All I have is my own personal experience, absolutely no real knowledge of the real facts or science behind it
I fought for over a year to get any kind of SPS growth, or really any growth to happen in my tank. I struggled to keep the big 3 parameters anywhere near optimal, usually one was always way out of whack. Then I read Sk8ters thread on RC and figured it couldn't hurt to scrap everything I'd been doing. I stopped dosing everything. I looked up BRS's dosing calculator and brought my Mg to 1400 over the period of a couple days. Then did nothing for a week but watch how that number fell and figured out my daily consumption. Dosed the tank again to get it back to 1400 and dialled in my 1.1ml doser and watched it for another week. Mg stayed rock solid. Then I did the exact same procedure for my Ca. After that I found my alk just dropped in line and followed everything except for just a minor dosing daily dosing to keep it topped up.
I've since moved to a Calcium reactor and have stopped dosing alk altogether. Been almost 2 years now and my numbers don't fluctuate at all.
I do top up my Ca in my water change water before I do it.
I fought and fought and fought to get good growth in my tank, but in my experience, it was only once my Mg was up, was I able to get any sort of stability in my tank.