Custom duty & cst chargability

Dear All 

I want not know the Chargability of CST 

For example

            Basic cost             1000

Plus  Custom Duty              50

          SUB Total               1050

          CST     2%               21

        Total                          1071

whether it was correct or not

Also if Custom exemption is available then on which amount CST charged whether inclusive of Custom duty or on Basic Price.