How To Use Web Income Calculators; Know Key Assumptions; "Monte Carlo' simulators give cautious advice, but smaller yearly payouts.(FUNDS & PERSONAL FINANCE)(FINANCIAL PLANS)

Byline: PAUL KATZEFF

Spending your money is perhaps the easiest part of personal finance.

But figuring out how much you can spend isn't always so simple.

"People often think if the stock market grows, say, 8% a year, then they can afford to take out that much," said Christine Fahlund, senior financial planner for T. Rowe Price. "But the market's annual performance is never exactly the same as its long-term average."

And a long downturn cuts deeply into account size. "Volatility makes it hard for an account to last as long as it would with smooth annual returns," she said.

To provide more cushion against severe market setbacks, T. …