Some differences in planning are personal. For instance, a couple planning to retire at 55 may need a paycheck-bridge strategy before they become eligible for Social Security at age 62, said Michael Kitces, director of financial planning for the Pinnacle Advisory Group in Columbia, Md. If the couple are in line for an inheritance or are planning to sell a second home, they can spend more earlier in retirement than might otherwise seem prudent, he added.

Other variations are economic. An extended period of stock-market gains may let some retirees increase their withdrawals. A couple of down years can have the opposite effect.

These factors must all be weighed in a paycheck replacement plan.

“The mathematics of this are beyond what most people could really be sophisticated at,” said Roger Ibbotson, a professor at the Yale School of Management and an expert on investments who has helped mutual fund companies advise retirees. “It’s not that they’re stupid. It’s just that it’s complicated.”

To give retirees some new tools, two of the biggest fund companies, Fidelity and Vanguard, have introduced mutual funds intended to make it easy for retirees to make systematic monthly withdrawals.

Fidelity’s Income Replacement funds use target dates to determine the period over which the portfolio will be drawn down. For instance, the Fidelity Income Replacement Fund 2038 is meant to provide retirees with monthly income for the next 30 years. The withdrawal rate starts at 5.01 percent in the first year and moves up gradually, to 6.30 percent in 2018 and 10.29 percent in 2028. In 2038, retirees get whatever is left, divided into 12 monthly installments, and the fund is depleted.