Odoo 16 - I'd like to be able to edit the totals for rows on Pivot tables.  Perhaps add a second total that's customized.  How can I do this?

Example, you can see in the screenshot the total "Untaxed Total" to the right?
I'd like to add another column that was that total minus the total in the Deliveries column.  I can't find the code in Odoo to inherit that produces the totals though.

Screenshot: 
https://pasteboard.co/S8cQOFPIkauo.png