OK, I'd just ignore magnesium for a while. If real, it is plenty high, and some folks have issues with magnesium testing results being inaccurate, so I would not sweat it. :)
Using Brs 2 part (just bought, didn't dose yet)
Current all/cal are 6.7 and 480.
Now one would tend to add just the soda Ashton get the alk up but I'm hearing that this is incorrect due to the see sawing of the two.
Using two part, how would you balance it out if you were shooting for 9.0?
I'd dose just the alk part for a while to get to 9 dKH, adding maybe 1 dKH per day and see what that does.
The calcium will slowly drop and when it gets to where you want it, start dosing the calcium in the same volume you dose the alk part and then monitor both and make adjustments based on the tank alk over time. Longer term, make smaller adjustments to the calcium dose away from the alk dose, if needed.
FYI, Reef Crystals commonly has over 1500 Mg levels so even not adding any extra it will probably take a long time to go down and may not ever go down under 1500. But many people run their Mg that high and higher with no apparent issues other then the possible sluggishness with snails which they seem to recover after a while. Like Randy said, I wouldn't worry about that level.