NEW YORK (MainStreet) — Landing a probe on a comet in the middle of the solar system is easier than calculating the savings required to generate a target retirement income. At least it seems that way. That's because so many investors – and advisors – rely on "back of the envelope" calculations, practically pulling a number out of thin air. If scientists had taken that approach, the little comet-exploring Rosetta spacecraft would have sailed right on past its target.

For years, advisors have relied on the "4% rule," the rule of thumb that dictates a 4% annual retirement income drawn from investment assets. If only it were that easy in reality.

“Common approaches like the ‘4% rule’ are easy to understand, but do not account for a client’s individual circumstances and can lead to unintended mistakes,” says Rod Greenshields, consulting director with Russell Investments. “We think advisors would do well to follow the lead taken by defined benefit plans and calculate a funded ratio. By determining the cost of a client’s liabilities compared to the value of their assets, the funded ratio offers a superior method of evaluating retirement readiness. The math behind the ratio is sophisticated, but the outcome is a simple yet powerful percentage that most clients understand immediately. This individualized approach gives advisors the opportunity to engage with clients in a meaningful conversation about their progress toward retirement goals and the amount of risk they may need to take in order to meet them.”

In a survey of advisors conducted by Russell, most advisors (52%) admitted that one of their top challenges in serving clients near or in retirement was “setting reasonable spending expectations" and “maintaining sustainable plans” (44%).

But only a few advisors make a serious effort to meet those challenges. Instead, one-quarter of the advisors surveyed said they base retirement spending strategies on pre-retirement expenses. In effect, predicting your future course based on where you have been. That's not a plan forward; it's a circle.

Nearly the same number (22%) said they use rule-of-thumb measures like the "4% rule." Neither strategy considers actual assets and lifetime liabilities as a part of a more comprehensive retirement income plan. In fact, only 16% of advisors surveyed take the more sophisticated planning approach.

When seeking advice regarding retirement income, look for an advisor that does more than ballpark calculations. If not, your investment plan may miss by more than a mile.

--Hal M. Bundrick is a Certified Financial Planner and contributor to MainStreet. Follow him on Twitter: @HalMBundrick

Read More: How Much Can You Really Spend Each Year of Retirement?