When you're trying to find the concentration of one substance as another substance you're talking gram equivalent weights.

The equivalent weight of an element can be derived by dividing an element's molar mass by the oxidation number (or, more conveniently, just Google search, "Equivalent weight of x" and you can usually find it. Also, here's a table of equivalent weights:

http://www.gewater.com/handbook/control_water_analyses/fig40-2.jsp). In the case of Calcium the equivalent weight is 20.04 (molar mass 40.08 oxidation number 2).

Once you have the equivalent weight of a substance expressing it as ppm as CaCO3 is the easy part. The equivalent weight of CaCO3 is 50 - that's why everything is expressed as CaCO3 - it's easy! It's like a lowest common denominator.

SO Calcium ppm * CaCO3 equivalent weight / Ca eq weight = Ca as CaCO3

Example:

200ppm Calcium in water expressed as CaCO3:

200 * (50/20) = Ca as CaCO3

200 * 2.5 = Ca as CaCO3

500 ppm Ca as CaCO3

so, yeah, that's right.