Adding a field to the Pivot view is not easy and probably not advised as the pivot view is changeable by many dimensions.

However, there are two easy way to do this:

- Export the Pivot as an .xlsx file and do the calculations manually in Excel/Google sheets.

        -    The issue here is the data will not update as it is updated in the system, which may be an issue for you

If you want to be able to view real-time data then you can link the pivot view to an Odoo spreadsheet. Add the column with whatever calculations you want and the data will update as there are changes in the system. 

More info here: https://www.youtube.com/watch?v=tPNqVfUB4zw