The recommended level is 1250-1350. so you're not really out of whack. If you have lots of coralline it will aid in the useup rate. (If you don't you may soon). If you stick with weekly water changes the level should subside.
Have you been adding any suppliments for Mag., or is this the level achieved by the salt mix? If it's just the salt, then the source for high Mag. is likely Oceanic. IO usually mixes to a Mag. level of 1170-1200. Maybe just use IO for your next couple of water changes.