A quote from Randy's article in Reef Keeping magazine. Since your Alk is already high..........
http://www.reefkeeping.com/issues/2004-05/rhf/
For these reasons, I suggest that aquarists maintain a calcium level between about 380 and 450 ppm. I also suggest using a balanced calcium and alkalinity additive system for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part additive systems.
If calcium is depleted and needs to be raised significantly, however, such a balanced additive is not a good choice since it will raise alkalinity too much. In that case, adding calcium chloride is a good method for raising calcium.