calcium vs alkalinity balance

hello, I have seen some calculators that claim the numbers are balanced. it says if your calcium is, lets say 440.... your dKH is 11.2

are these estimates or are they accurate.

I only ask because my calcium is 440 but my dKH is 9.0 bases on my Red Sea test kit. I have been testing a few days and not once have my numbers agreed with the calculator

http://www.ultimatereef.net/pages/calcium_alkalinity/

I only ask because i want to know if i need to replace my test kit.

thanks in advance