I find a lot depennds on what salt you use.
If your salt is low on magnesium (and yes, I consider 1200 ppm to be low, NSW is closer to 1380 or so) then you need to add a lot to your water used for water changes.
If your salt is good, but your magensium still slides due to lower water change schedule/heavy use of kalk then you can either manually dose or do some of the other methods like adding media to your calcium reactor.
Don't look to calcium reactor additions to raise your magnesium, you will grow old waiting.
FWIW manual additions can be the most economical.
