Be aware: the linked Salifert Mg test is for marine water only! I did not notice that, and it is really not working with fresh water.
With a reporting resolution of 30 ppm Mg, that test would be of limited use. Of course, if your tap water had Mg levels higher than that, it could be useful to know.
With the preferred limit of 40 ppm Mg in brewing water supplies, its the resolution that makes that test kit less useful.