It would depend on density as well. Are people standing shoulder to shoulder solid?
I think the Beck rally people range from equally to 1/3 as densely packed as the stadium people.
The 110,000 gameday total includes a lot of people working under the stands and elsewhere - I used 100,000 for a simple round figure.
The density will also vary with the temperature. You get a nice cold day in January and you can have a crowd 20 deep that's only 3 ft thick. You get a hot day in July, and they'll be stretched out a lot just so they can breathe.
Plus, since this rally was held at the West end of the Mall you really need to find out how many were UNDER THE TREES.
There are definately areas of grass showing through. I was half tempted to photoshop some more people but then thought better of it. Would not be good to have that pic out there as amunition for DUmmies.