Can you spell this out in more detail?
I believe you are describing a calculation field that returns text ("Net 30 days"). There are several ways this field could be defined. If you post the actual definition here, it would help.
I'm not sure what you mean by "Auto Populating with the Field I have selected,". Do you want the calculation to display the contents of a field instead of the text "net 30 days" or something else?
It may be as simple as changing your calculation to refer to the name of a field in the same place you find the text "net 30 days".