Hi Melissa,
what do you like to do with a "percentage of empty space"?
And how would you like to define what is empty and what is not?
I could imagine that one would export a pixel image of a given page.
Or better, export a pixel image of the rectangular area of your page margins.
Then open the image in PhotoShop and do a selection on the pixel contents.
One could calculate from the area of that selection how much space is left around the selection.
That would only work if a text frame has no strokes applied or if a text frame is not filled with color.
Note: The algorithm above is just one sample how one could do it.
Don't know how your specs are or if it could meet your expectations.
All in all that would require writing a paid custom script…
Regards,
Uwe Laubender
( Adobe Community Professional )