Interesting math there, but since the air ways are going to be grounded till tomorrow afternoon, I don't think the terrorists will have the opportunity again... At least until the airlines get back to a normal routine scheduling.
They were smart in choosing flights that (assuming they were non-stop) fully loaded with fuel, since they were meant to fly cross-country. Since their targets were relatively near by from there they took off, the fuel on those jets were basically full, to cause as much damage as possible.
As for the one flight that crashed in PA. I'm thinking that was a hijacking that went bad. Perhaps the crew, or passengers were able to kill the hijackers and ended up grounding the jet. If this were the case then the crew and passengers should be considered heroes
for stopping them from possibly crashing into another target.