To be clear - there is no magical number that the numbers add up to.  It is arbitrary.  Move that rate up and some people lose more while others gain more.  Now, technically if you moved it too high then it would be past the point of liquidation for those that shorted and you would be short on funds to payout those that went long.

But Bitcoinica had a good month of stability in btc price to payout and they failed to do so - so they should have to pay the difference.  And if they don't have the funds then the loss is spread between those in long positions.

But the bottom line is that the exchange rate should be the rate at the time that the money is paid out.  Either that or they take the highest rate between then and when paid out for long positions and the lowest rate between then and when paid out for short positions and close at those rates and then spread the loss between all those that held positions (as a percentage of the gains made from their positions).

Zhou had mentioned possibly picking a number in that range before (if he were running the show) because he figured that the payouts would happen as quickly as possible so any price change would be insignificant.

Bitcoinica seems to have chosen that number because presumably it was more favorable for those that were buddy-buddy with Tihan.  I can't think of any other reason to do so.