Shoot, I missed your water volume estimate. I was adding 15 times 15ml in my *40* tank to raise it 100 when I was fighting low Mg (bad salt, this is why I say test every bucket, just because you paid more for it apparently doesn't mean squat for quality control
)
Anyhow basically you're gonna be adding a whole lot of Mg. My suggestion to start with one tablespoon, yeah, if you do that, you're not even going to notice a bump in the levels the next day.
But the principle remains sound, start small and work your way up to an amount that raises it say 50 to 100 and leave it at that per day until it's at 1400 or wherever you're going with it.