Ahhh yes, the endless pursuit of the aquarist to maintain super high calcium with mutually excessive alkalinity. If you are keeping SPS or Clams you should maintain calcium in the lower 400's. I would say 400-420 otherwise 350-400 for any LPS. The reason that you are having trouble with DKH is that your calcium is so high. If you try and raise your Alkalinity to high while mainting high calcium, you could cause calcium to precipitate. What this means is that you will see a "snowstorm" effect in in your water because it's lost it's ability to absorb calcium as a useful substance. Just trust me, lower your calcium to the lower 400's and then work on raising your alkalinty with a KH buffer.