I have a Database (list) that contains projection and actuals and can get a nice subtotal as needed. I would like the grand total to be the difference between the subtotals, not the sum...
1. Using Excel Range/Table as data source
2. Standard PivotTable, but made using data model
3. Using PowerPivot data model
Also, what version of Excel do you use for this?
It would help if you can upload sample workbook that mirrors your set up (with sanitized values). And also put manually created expected output in the workbook.
Unfortunately, you can't do it in PivotTable alone in this case.
What I would do... is to add calculation above the pivot table (to avoid over-writing formula when pivot range is extended).
How dynamic do you need this to be? Should it extend to entire column(s) when pivot is fully extended? Or just needed for Monthly level?
Using PowerPivot, you can create dynamic measures that will appropriately fill each field.