Don't waste too much time mulling over those retirement rules of thumb that seem to pop up everywhere. They've never been very useful, but changes in the way people live and work make them even less so.

Here are a couple of rules debunked.

- That 80 percent spending maxim. Once you start planning for retirement, you usually run across this rule: You'll need roughly 80 percent of your pre-retirement income once you retire.

That's a ridiculous figure, pulled out of thin air. Investment companies say they often use that number to help people who don't have any idea of how to plan their retirement savings. But it mainly helps to scare people into thinking they have no hope of saving enough for retirement.

In truth, they may spend more than that in the first couple of years after they stop working, but they're likely to spend far less, on average, over their entire retirement period. By the time they are 75, they're likely to be spending about half of what they did when they were 50, according to the Labor Department's survey of consumer spending.

To the extent that 80 percent figure is worthwhile, it's meant to compare your last working year with your first retired year. So younger people, who still may be paying for kids in college, have high incomes with high tax rates, live in expensive homes with high property taxes and the like, shouldn't even bother with this figure.

Instead, take a look at your monthly expenses and see how many of them will follow you into retirement.

Include your car payment; most people buy at least one new car after they retire. Add more for medical and dental care that is likely to cost more as you age. Bulk up travel expenses, if you like.

Eliminate all child-related expenses, and reduce grocery bills and clothing costs. Also cut tax payments if you expect to pay less in taxes once you're no longer working.

Multiply this seat-of-the-pants retirement spending guesstimate by 12, and you'll see what you might expect to spend in a year, in today's dollars, once you retire.

- The asset allocation rule. Many planners also tell people to subtract their age from 110 and use the remainder as the percentage of their nest egg that they should have in the stock market.

That would mean that by the time a person is 70, he or she should have 60 percent of holdings tied up in bonds and money market funds. But that's a gross generalization that probably wouldn't work for most people.

In fact, large mutual fund companies that are running one-decision target retirement mutual funds are finding that they have to be more aggressive with stock investments than originally expected if investors are to meet their retirement goals and have their money last a lifetime.

Some people may have guaranteed pensions or money in annuities that would allow them to be more aggressive than they otherwise would be with the money they invest themselves. Others may have money earmarked for children or grandchildren and want to invest part of their portfolio aggressively.

Others may have very old stock holdings that would result in large capital gains taxes if they sold them suddenly to move more money into bonds.

The question of how to split up your assets is best not left to a simplistic formula. Each individual and couple should make that decision based on how much they have, how much of it they're spending every year, what they intend to do with their money, and their personal risk tolerance.

If you have to hire a financial planner to figure that out for you, it would be money well spent to get a better answer than the rules of thumb would ever deliver.