How Much Can You Withdraw From Your Savings
(January/February 2013 Money magazine (page 116) -- No Link)

"Best move: Recalculate your withdrawals every year to take into account your current account balances and the fact that your nest egg doesn't have to support you for as long." 

"With a decision this big, you don't want to blindly stick to the 4% rule or any other rigid system..." 
 
"As a practical matter, though, recalculating your withdrawal rate this way can be quite complicated.  So unless you're working with a financial planner capable of doing the number crunching for you, your best bet is to go to  an online tool like T. Rowe Price's Retirement Income Calculator every year, plug in your most up-to-date information, and adjust your withdrawals up or down as necessary." 

I couldn't agree more with this advice from Money magazine.  And the online tool "like" T. Rowe Price's that you should use is located right here in this website.  See related link below for a discussion of some of the weaknesses of the T. Rowe Price tool.