This question has provided some interesting replies! Good one.

I run a Kalk reactor during night hours only, and run a Ca reactor continuously. Both are controlled with pH meters and peristaltic pumps linked to the controller. These supply all my Ca/KH requirements.

I test Alk once per week (unless I'm having some problems). Maybe test Ca monthly, but I generally assume it is linked to Alk. I might test Mg only every 3 months. I've never had to dose Mg, which I cannot explain.

yeah this applies to me as well, i use reefers best salt, so maybe thats why..?

Doesn't explain it for me...I've used a variety of salts. Weekly 10% water changes, possibly?

I guess there is enough mg in the seachem advantage calcium that my reef needs.
and i do a 10% wc, every two weeks.

Since you run a Ca Reactor, chances are that the media you use has a good proportion of Mg to Ca... if you're using crushed coral skeleton as your media, that should have the perfect balance of everything that stony corals need, right? (My thinking might be off here, but that's what I always thought...)