Hi David! Unfortunately, that isn't really how Tableau works. Excel is way more flexible than Tableau is, especially when it comes to editing individual cells. While you can micromanage cell contents in Excel, you can't in Tableau. Tableau's cells are more like products in a chemical reaction: you can use different reactants and processes to create different batches of chemicals, but you can't really change individual molecules.
The easiest way to give you what you want is just to create a separate worksheet for the difference calculations and put it underneath this one in a dashboard. Otherwise, you could try enabling grand totals and using some fancy table calculation tricks to fine-tune what the grand total displays.