1700 while high is not out of hand. I would not dose any MAG and you should see it deplete and having good water changes will help. There is a thread about battleing briopsis with High Mag. You are at the very bottom of the range for this though. they say setting it at the 2200 level or so. Mag will slowly decline some salts start with pretty high mag becuse many people rely on water changes for mag suplimenation. Mag does have a critical role in keeping the stability of alk and calcium I would refrence Randy Holmes-Farley Aquarium Chemistry: Magnesium In Reef Aquaria — Advanced Aquarist | Aquarist Magazine and Blog I would say read the bible of mag in the aquarium.
If it is the new version red sea test, they have been reported to be off by a lot. 300 or so, in some cases. All the other red sea tests are good, but the mag is not getting good reviews fyi.