FireCalc does exactly that using the option at the bottom of the Spending Model page. You can vary your withdrawals on a year to year basis. I am in your situation, so I just enter a small or 0 withdrawal for years 1 - 5, then the full withdrawal thereafter.
Of course to see that feature you need to have sent in a donation to the author. You did send in your supporter payment, right
San Francisco Area
ESR'd March 2010. FIRE'd January 2011.
As if you didn't know..If the above message contains medical content, it's NOT intended as advice, and may not be accurate, applicable or sufficient. Don't rely on it for any purpose. Consult your own doctor for all medical advice.