When filtering the layout by let's say the top customers, I'd like to know how much am I missing. Right now using "Keep Totals" I can infer the rest of the values. But that is a tedious task, best done in excel.

I'd like to have an option "add remaining sum" that would create a new row above the totals' row. The content would essentially be the totals minus the above rows.
It could look like this:

That would show how relevant the selection is in comparison with the total value.