If you think figuring out how much money to put into your retirement savings accounts is complex, wait until it's time to take money out.

That is emotionally challenging to people who have spent their lifetimes trying to build their nest eggs, but it is also a mathematical challenge. If you take too much out, you risk running out of money just when you might need it most to pay for care - a grim prospect. But withdrawals that are too low may not leave you enough to pay your bills or enjoy that retirement you've been saving for.

Much of the energy in the retirement planning community now is being devoted to helping workers figure out that magic number in advance: If you know how much you want to spend, and you can calculate how much of a nest egg it takes to generate that income, then you know exactly how much to save.

The Labor Department has been preparing a rule that would require employers to provide 401(k) participants with lifetime income illustrations that translate their account balances into future monthly checks. The concept has broad support from everyone from the Pension Rights Center, a pro-labor group, to the insurance industry.

The issue is how to do that calculation, and how far to go with it. An illustration based on an annuity formula would show you a much higher income stream than one based on an independent "safe withdrawal rate" rule of thumb. Doing a back of the envelope plan for how much you think you can withdraw is a far cry from turning your retirement account over to a company that says it can guarantee that income stream. You wouldn't necessarily want to count on a paycheck illustration that was calculated one way and then do something completely different with your money.

If you are managing your own money, conventional wisdom holds that you can pull 4 percent a year out of your savings, and increase that by about 3 percent a year for inflation. Here's some math for a 58 year old woman with $150,000 in a 401(k) today(an amount Fidelity Investments says is about average) who expects to retire at 66: Invested at 7.5 percent, your retirement account should grow to $249,000 by the time you retire. If you pull 4 percent out, that would get you a $830 monthly check in the first year.

If you bought an annuity (at today's interest rates for a 65 year old woman) for $249,000, you could pull $1,357 a month from it, according to ImmediateAnnuities.com. That's a 6.54 percent withdrawal rate - but of course you would give up the principal, so you wouldn't have that money available for future emergencies, gifts, special occasions or cars. It wouldn't rise with inflation, and while you would get that guaranteed income as long as you lived, there would be nothing to leave to heirs.

Most big investment companies offer some kind of retirement calculator that tells you how much income you can take in retirement. Firms like Vanguard and Fidelity are going farther - they offer mutual funds aimed at retirees in the withdrawal phase of life. These funds offer no annuity-style guarantees, but they manage the money with the expectation that it would last a lifetime, and offer higher payouts than the 4 percent rule would allow.

If you invest $249,000 in the Fidelity Income Replacement 2042 fund, you would start with $965 a month in withdrawals that would be designed to keep pace with inflation. If you invest $249,000 in the Vanguard Managed Payout funds, you get a choice of three different initial monthly payouts: $524, $937 or $1,397. The less you take out initially, the more of a nest egg you can expect to leave to your heirs.

Now fund company BlackRock Inc has added another innovation that bridges the gap between annuities and independent investments. It has built a bond index (that it expects to use as the basis for mutual funds) that allows investors to peg their current savings to their future monthly income. It's simple web-based calculator (at) reckons that today's 58 year old with $150,000 who plans to retire at 65 will be able to withdraw $860 a month then, and inflate that amount by 2.5 percent a year.

Once BlackRock gets approval to market bond funds based on that index, it expects to tell investors that they should be able to count on buying an annuity at retirement age that gets them that withdrawal rate. Developed by Chip Castille, a managing director and one of the creators of the original target-date fund, the bond-heavy new product is a safety play for investors, but it's not so drastic as giving up income to buy an annuity.

Investors who resist taking an all-or-nothing approach to how they withdraw their retirement income probably will be happiest in the long haul. As you approach retirement, figure out how much you really need to collect every month after you've subtracted your Social Security benefit, and consider one of these targeted products for the portion of your savings that you can devote to that.