Among the more difficult questions in personal finance for the near-retiree is how much to assume you'll take out each year from retirement savings. The typical starting place is the so-called 4% rule of thumb.

Financial planner William Bengen in the early 1990s took a detailed look back at market history to figure out a "safe withdrawal rate." That's the maximum that can be taken out annually from retirement savings without worrying about running out of money over a 30-year period.

Subsequent 4% withdrawals are adjusted for inflation. Among his assumptions were a portfolio of 50% stocks and 50% intermediate-term Treasuries, and a 30-year time horizon.

The investment platform Morningstar recently came out with a research report that challenges the 4% rule. In "The State of Retirement Income: Safe Withdrawal Rates," the authors calculate that the suggested starting place should now be 3.3%, with the same assumptions.

The difference matters. A 4% withdrawal on a $250,000 portfolio is $10,000 while 3.3% is only $8,250. The authors believe retirees are unlikely to receive the kinds of returns they did in the past. The factors driving their lowered expectations include high stock market valuations relative to historic norms and low interest rates.

The 3.3% number is the headline result that has garnered much attention. However, far more interesting is their discussion about ways to adjust that figure. For example, the retiree who delays retirement by one year can use a 3.5% rule.

Someone delaying retirement by five years can spend more than 4% (using the same underlying assumptions). Similarly, you can be more aggressive with withdrawals if you're disciplined enough to cut back when markets are weak. Retirees who don't want to leave an inheritance to family members or charity can take out more every year.

Of course, the report only deals with the money side of the equation. "The other half is about planning your retirement activities so that you have a high-quality retirement life," writes retirement coach Mark Fischer in his Serious About Retiring newsletter.

That's why it pays to take your time, run the numbers (from budget to withdrawal rates) and think about what matters to you and your loved ones.

Planning ahead eases the path to one of the more complicated life decisions you'll ever make.

Chris Farrell is senior economics contributor for American Public Media's "Marketplace" and a commentator on Minnesota Public Radio.