Possibly - see Jonathan Drummey's write up here:Customizing Grand Totals – Part 2 | Drawing with Numbers
I'm guessing the fairly straight forward example above is not representative of the real data though. Another possibility is to use the calculated field in a different view and align two views in a dashboard. Jonathan describes this in the Part 1 of the series on that page.
I do that in a bunch of places. (I use ... IF FIRST()=LAST() then [calc field]... to identify the total row. But it's the same principle.)
So roughly, the sort of calc field I would use to display what is in the initial post would look like this:
IF FIRST()=LAST() then [calc field] else
And [Calc field] would be whatever calc is needed to be displayed in the grand total cell.
Quick question, because it could change the scope of how this is done.
Do you want to the Grand Total to be a String? I.e. it would literally show '(100/40)'?