To require that a single marking column calculation be used for all classes:
1. Log in as Supervisor.
2. Go to Setup, Define Calculations.
3. Mark the Faculty to Automatically use supervisor calculations option.

To apply this change to current grades, clear the server cache and force the system to recalculate the marking column by:
 
Select one of the marking columns used in the calculation,
Click Setup, and then click Categories,
Change the factor to a different number for one of the categories. When the dialog box asking if you want to continue appears, click Yes, then change the factor back.

This will recalculate the marking column grade, then change it back, which forces the system to recalculate the calculated marking column. You can do the same thing by manually retyping one of the grades used in the calculation for each student.