That sounds backwards unless you have a heavy SPS population? Alk would normally deplete faster than Ca. 1 mEq/l alk per 20 ppm Ca on average. I would recheck things just to be sure.
What levels do you normally maintain each (alk/Ca) at, how often are you doing water changes/what volume and what are the chem stats for the salt you use?
What water volume is the tank?
If all checks out and you truely do have a faster Ca depletion, you don't want a two part chem or kalkwasser. Kalk replenishes both alk and Ca, not just one side of the equation. You just need a stand alone CaCl additive (dry preferabley) if that's the case.
Cheers
Steve