Don't! You'll more than likely just create a solid lump of calcium carbonate, depending on how much you add at once. Just as an aside - never add salt mix to an aquarium, unless it's a brand new tank full of RODI that you're setting up for the first time.
RTN = rapid tissue necrosis. Probably most associated with acropora; coral looks fine, if a little pale, one day, and is a dead skeleton the next.
What jda notes is true - lots of folks ran really high alkalinity 15 - 20 years ago. There's a couple of reasons that they got away with it (I was one, btw). First, nutrient-stripping substances like GFO, Lanthanum Chloride and procedures like carbon dosing to lower nitrates weren't common, and people typically didn't test for phosphate. Most tanks had a good bit of algae and relatively high nitrates and phosphates, and while nitrate was (inappropriately) viewed as bad, there wasn't much reefers could do about it except lots of water changes, which only lowered it some. The second reason is that acropora species weren't common in aquariums, as they just didn't live all that long. In fact, one of the reasons that Julian Sprung has the reputation that he does is that he actually got acropora species to grow on the glass of his 20 gallon reef in the late 1980's - that was considered impossible but a whole lot of experts, including many scientists. That said, some stony corals were quite common, and a lot of us had good success with them, particularly montiporas, turbinaria, so-called "pagoda corals", and euphyllias.
Another reason that people ran high alkalinity was that's the way "salt" came, which back in the day was almost universally Instant Ocean. Dosing was relatively uncommon, and surprisingly, the most often dosed compound was calcium; alkalinity was pretty much ignored. Today we know better - alkalinity stability is absolutely critical for success with acropora, and calcium doesn't matter all that much as long as it's >380 ppm.
I find your comment that you only do water changes once per month and you still have alkalinity > 10 dKH surprising. Most aquariums will "lose" alkalinity over time much more rapidly than that, even if it doesn't have much in the way of stony corals in it. In any event, I'd monitor it rather closely - if you've very few corals in the tank, I'd check it twice a week with a high-quality test kit (Salifert or Red Sea). There's a lot of folks that will argue back and forth about what the ideal alkalinity is in an SPS tank, but most of them will agree that stability of whatever target they pick is fairly important.
