It depends on what type of corals you plan on having. Calcium, alkalinity and magnesium go hand in hand and need to be balanced. Having a high mg level and low calcium will not get you the results you desire.
Sps coral frags, clams - 12dkh, calcium 465, and mg 1390
Soft corals and large sps, 8dkh, calcium430, and mg 1280