Calcium wouldn't drop at all when you first added the baking soda.
When that alkalinity is consumed by corals or through precipitation, calcium will drop by about 18-20 ppm Ca++ for each 1 meq/L of alkalinity.
The exact ratio for calcium carbonate is 20 ppm Ca++ per 1 meq/l of alkalinity, but since magnesium and other ions can get into the CaCO3 in the place of calcium, the number is a tad under 20 ppm.
I discuss such issues in this article:
When Do Calcium and Alkalinity Demand Not Exactly Balance?
http://reefkeeping.com/issues/2004-12/rhf/index.htm