Grand Summary Formula Column

Grand Summary Formula Column

Hello.  I want to put a formula in a summary row, and have no formula elsewhere.

Specifically I want something that does this.  See how the Three-Day Average Column has 350 in the "My Movie" Summary line, and is determined by dividing 3-Day Gross / Screens.

I've tried everything I can figure out to try.  I won't bore you with what I have tried.  What works?

Title Venue Three-Day Average  Screens 3-Day Gross
My Movie Astor Theatre 1 567
Average Theatre 1 133
My Movie 350 2 700
Grand Summary: 2 700