Where did you get that line "A 5,500 official attendance is far less impressive when only 4000 people actually are there to see the show"? What I said was that, in recent years, the number of people actually attending -- by which I mean the number of individual human beings who were present for any part of the five days of the convention, and what I think you'd call "official attendance" -- is a figure of around 5500 +/- 1500. Yes, it's a huge uncertainty, and that's why Worldcon runners get ulcers. And furthermore, with no financial backstop and catastrophe if you overshoot expenses, we tend to build in far higher contingency amounts than a typical annual ongoing convention would have as a percentage of revenue. If Worldcon was an ongoing organization, it could build up a cushion against bad years, but it can't do that, and that probably adds at least $100K additional expense (around $20/membership) to each convention.

(And you couldn't impose such a financial structure on Worldcons by setting up a separate contingency fund, because every Worldcon would have a maximum incentive to draw on the fund and a minimum incentive to donate anything to it. "Why should I give you anything? It's not going to help my organization -- our Worldcon was last year.")

As far as trying to quanitify the uncertainy goes: the Worldcons for which we have figures suggest that the number of at-door members is around 10% of the pre-con registration, but again, it's a very rough number. Worldcons move around so much, and each area's demographics are so different, that it's extremely difficult to make meaningful generalizations.