Multiply 0.57 percent (the figure in the fourth column) by each $10,000 you have accumulated. That equals 5.7 percent (10 x 10,000 if you have $100,000 saved.) Next subtract the total (5.7 percent) from that 17.6 percent savings rate to get 11.9 percent.

This study takes Social Security income into account, which replaces a higher percentage of income for low-wage earners, and a lower percentage of income for high-wage earners. (To simplify their calculations, the authors assume that full benefits are available at age 65, but in reality, the study's authors recommend that you wait until full retirement age to collect the full benefit.)

Of course, if your salary increases in the future, you'll need to make adjustments. The numbers aren't foolproof, but these rates have a 90 percent chance of success, say the authors. They ran each combination of age, income level and savings rates through 2,000 market scenarios in "Monte Carlo simulations" -- altogether 72 million simulations -- to get these numbers. The portfolios they used in their research resembled those of target-date funds, which become increasingly conservative as retirement draws near.

Notice that the savings rate trends upward as you get older. That's why it's easier to meet your retirement goals if you get an early start. But as the chart shows, it's never too late to get started.