I must tel you, Cornelius'formula guide was very helpful for me. I just passed my PMP exam last week friday (August 28th, 2009). I will post my lessons learned shortly.
I can't give you an exact percentage. I know I had around 10 or so, could be +/- 2. I was happy to get that many, calculations are much easier than the scenario based stuff...you either know the answer or you don't.
EDIT TO ADD: I purchased the Formula Study Guide from Cornelius and crew and found it to be invaluable.
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