According to the parameter list for new salt water that I see referenced frequently Reef Crystals will mix up to 1440 ppm for magnesium.yes thoroughly mix before testing. Rc is known to have high mag, so it wouldn't really surprise me if your not in the 1500.
It's usually not an issue till it gets seriously high, then just inverts have problems.
I may test my new saltwater this evening and see if I get around that value, since I also use reef crystals.