For the sake of comparison, I went to the Gencon Indy site and browsed around. Courtesy of the event guide in CSV format, I ran the same numbers for the whole convention. Here is some interesting information for you.
Maximum number of tickets for events: 140,235
Number of hours of gaming: 17,830.5
That's a lot of gaming.
And then I found the "Tickets Available" column and ran a quick calculation.
Number of tickets still available: 90,455
So the occupancy for the whole of Gencon Indy at the time the CSV file was last generated on the day that the convention opened is 35%.
Even with such a large convention, there are still a lot of people waiting until the last minute to get into a game. The 18% for indie games on demand looks even better than I had previously thought.