Frankly if you don't have lots of calcium using corals you can go down to about 350 and it won't matter a toot except your coralline won't grow so good. If you do water changes it won't go lower than that.
Be realistic. Whay are you adding calcium? What's using it, and how much? If you have a tank with jsut fish and softies, you have no need to dose calcium at all. And frankly just adding calcium willynilly is more likely to give you problems than not adding it.
Because if you start adding calcium you had better keep a close eye on your alk and possibly magnesium as well as artificially cranking up calcium alone to force coralline growth will deplete those other too, leading to a pH crash.