geez people, 12.3 kh is not that bad, some salt mixes come that high.
It's not the end of the world.
And yes, 640 ca is high, but again, not the end of the world.
I wouldn't do any crazy 50% water changes, just keep up your regular schedule and it will come down.
I had a dosing incident, ca went up over 600.
Was that way for weeks before I noticed. I let it come down naturally, didn't lose a thing, fish or coral.
I would verify your salinity though.
Don't know what you are using to check, but if it's a swing arm, and it's off, your ca & kh will be off.
And if you're using a swing arm, get a refractometer and some pinpoint solution.