Hi Kevin,
Ahh okay gotcha. So as far as the calculation goes that depends upon the field type. You assign this at the same place I mentioned earlier IMG --> Personnel Management --> Enterprise Compensation Management --> Compensation Administration --> Settings for User Interface and Process --> User Interface for Compensation Planning --> Planning Table --> Define Structure of Columns on Tab.
Here is a proof of concept showing you before and after and how it shows up in a sandbox system with a custom field.
Before --> does not show up in calculation box
After --> I set the data field under the assigned fields (see screenshot below).You can see it now shows up under the calculation section. Make sure the column is not hidden on the screen or anything.
Thanks,
Imran