McBee's calculation is correct for events with one winner.
If there are more than one winner possible (for example Horse Racing Place Markets) then the calculation is different.
The following has been copied from Betfair's Rules and Regulations:
By way of another example, assume there is an outright winner in a golf tournament but 7 players tie for 2nd place. In the "top 5 finish" market, after settling on the outright winner in the top 5 market there would be 4 other designated winners places available. 'Client A' has backed one of the winners for 300 at the traded price of 4.0 and 'Client B' has taken the other side of this bet. When the event is settled, the stake (300) is multiplied by 4/7 (i.e. the number of expected winners (4) divided by the number of actual winners (7)) to calculate the reduced stake (171.43) and the remainder given to the layer (128.57). The backer then receives the traded price matched (4.0) multiplied by the reduced stake (4 x 171.43 = 685.72). In this example, Client A's net winnings are 385.72 (685.72 payout minus the original 300 stake), and Client B's net losses are 385.72.
Cheers,
bamthwok