Thought that a low Mg will limit your Ca, so what I've been doing if mixing new IO (low in Ca and Mg) is boost the Mg first then and the Ca supplement. If I ever need to adjust my system, been following the same order (Mg then Ca).
My belief is somewhat re-enforced by this article
(What is that Precipitate in My Reef Aquarium?) or at least my understanding of it, though probably really doesn't matter what order since not running my Ca or Alk overly high.
Would also suggest raising levels over a few days. Seen this
calculator?