First, I'm sure you've studied the foundational piece
on customizing Grand Totals by Jonathan Drummey:
Following it's first repecie of finding a Grand Total row
(the MIN != MAX one) one could place a custom calc in it.
So do I (please find in the attached on Sheet 7).
The problem is to place such Grand Total row(s) beneath the main calcs.
It needs a special (Dummy) dimension with two values (1 and 2),
so I made it with copying your datasource table into Excel file (twice)
and UNION'ing the two identical sheets in Tableau (hence version 9.3).
With the [Sheet] as my Dummy dimension I am able to make the layout.