that system is old...On the website it is:
This 25 BTC is divided among all of the users that contributed to that round, weighted by the number of shares that they earned. Therefore, the reward earned by a given user is given by the following formula:
(25 BTC + block fees - 2% fee) * (shares found by user's workers) / (total shares in current round)
which is what is being used now
That's not right. If you do the math on any of your rounds in the statistic page you'll see it doesn't match your formula. And in fact, right beneath the paragraph on the website that describes the formula you posted, he says "In fact the formula above is simplified. The real math behind the payout system is described here" and points to the same forum post I linked to.