Question about custom fields using Pivot Tables.

Question about custom fields using Pivot Tables.

I have created a pivot table showing annual revenue of a client and how much payment that client is paying my company. Is there a way using pivot table to add an additional field that subtracts those to fields / shows me a percentage of that difference?  I know this is possible using formulas but would like to be able to do it using a pivot table. Currently it only allows me to to add fields based on the original scope of the table. Would I have to create a new field with that formula or something?