I found a similar thread:
http://www.homebrewtalk.com/f128/total-hardness-calcium-magnesium-153586/
With a link to Palmer's Site:
http://howtobrew.com/section3/chapter15-1.html
I guess total hardness is not equal to Ca (ppm) and Mg (ppm):
(Ca (ppm)/20 + Mg (ppm)/12.1) x 50 = Total Hardness as CaCO3
My units are in mg/L on my analysis, with if you talk ions in equal to ppm, but not when talking molecules (CaCO3). So I believe the below statements are true, but I'm just guessing at this point:
Ca (ppm) + Mg (ppm) = Total Hardness as CaCO3 (ppm)
Or
Ca (mg/L) + Mg (mg/L) + CO3 (mg/L) = Total Hardness as CaCO3 (mg/L)