As a standalone without controller i find my jabeo dp4 acceptable, doesnt have all the bells and whistles of some others but i find it accurate enough and easy enough to calibrate for my use of alk,ca,mg and iron. And at a reasonable cost. If you ever get a controller there is no way to connect it to it.
If you are just doing calcium i see possible reasons,
-your salt mix is calcium deficient. Dont need a doser just mix calcium into your mix water to get desired ca level. Or use a higher calcium salt mix
- you also need alk and there is some test error. I find it unlikely your system is using ca but not alk. kalkwauser might be another way to go, only need an ato and it will add alk and ca, as well as keep water level.