There are way too many questions for us to ask based on what little you've given us. We need more information on your tank, methods and tests before we can give you an informed answer. Actual numbers, what you currently keep and what you currently add would be a good start. These articles will also answer your question if you have a little free time.
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH
http://reefkeeping.com/issues/2006-06/rhf/index.php
When Do Calcium and Alkalinity Demand Not Exactly Balance?
http://reefkeeping.com/issues/2004-12/rhf/index.htm
An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System
http://reefkeeping.com/issues/2006-02/rhf/index.php