THat is true.
Your low mg level is a reason you wouldn't have coralline. Get it to 1350 and if you have any, you'll start seeing it develop.
To answer your original question, btw, the best procedure is to dose only as much as the instructions say, wait 8 hours for it to fully dissolve and work its chemistry---then test again, add again. Bring your magnesium up first to 1350, because it is the stabilizer of the other two. Then start working on your alkalinity. Same routine. Get it to 8.3. Once you've got both of those behaving themselves, bring your calcium up to 420. NOW---add 1-2 teaspoons of kalk per gallon of water in your autotopoff reservoir, and just let topoff water supply THAT to your tank. The addition of that calcium powder into the situation will keep that ratio of readings locked so long as the topoff water keeps coming AND so long as your magnesium stays above 1200. That is the 'floor' of the readings. Keep it up around 1350 and your other two readings will stay where you put them so long as the kalk-water mix keeps coming in as topoff. Kalk is how I supply the stony coral in my tank, and it has been a standard method since, oh, the 1980's. Kalk is also cheap. In the states, I use Mrs. Wages Pickling Lime, which is literally used to make pickles, no joking.