For me it's impossible to manage the calcium and alk levels only by waterchanges in a SPS-tank. In a softcoral tank it might be possible, but in a SPS tank it wouldn't be.
I change the water two times a week (every time 5 %) - that is 40 % at month. And I still need 100 ml calcium, 100 ml carbonats at day.
I use the "balling method" to increase the calcium and alk levels.
Sometimes I put AragaMilk and Purple Up in my tank, some natural calcium additives.
But the most part comes from the "balling method" (Calciumchlorid, Natriumhydrogencarbonat and NaCl free Mineral Salt)
Greetings
Sudad