It seems the new mantra is that you need 100% or more of your pre-retirement income to live comfortably in retirement. But shouldn't you be able to get by with much less considering you no longer need to save for retirement and you may no longer have a mortgage? -- David Kolkebeck, Readsboro, Vt.

I agree that the "replacement ratio," or percentage of pre-retirement income advisers claim one needs to live well in retirement, seems to have crept up over the years.

When I started writing for MONEY magazine in the 1980s, people routinely referred to the "70% rule" when discussing the percentage of pre-retirement income one ought to shoot for.

Soon 70% expanded to 80% or 90%, and before long, some people began suggesting that 100% was a more realistic benchmark. A few years ago, one retirement research firm even came up with a figure of 126%.

Related: Can you trust your financial adviser?

Conspiracy theorists see something sinister in this retirement income inflation: Ah, those sly investment firms are pushing the replacement ratio higher, so we have to invest more money in mutual funds and the like, which allows them to collect bigger fees. But I think something less ominous is at work: uncertainty.

The simple fact is that it's tough to know exactly how much income you'll actually need in retirement. A recent study by the Employee Benefit Research Institute found that household expenditures decline later in life, dropping roughly by a third between ages 65 and 85.

But that doesn't mean your spending will follow that trajectory. Your outlays in retirement can vary widely depending on a number of factors, including: how closely you want your retirement lifestyle to mirror the way you lived during your career; how much you've saved for retirement and your kids' college expenses; whether you retire with a mortgage or other debt; whether you have retiree medical coverage from your company or will rely solely on Medicare; and how much you earned during your career.

So rather than viewing a replacement percentage as something you can pinpoint precisely, think about it as a very general guideline that can help you estimate how much you ought to be saving during your career and to gauge whether you're making progress on the retirement planning front.

As a baseline, I'd start with a replacement ratio of 70% or 75%. That should reflect the fact that many of the expenditures you face during your career -- saving for retirement, work-related expenses, the cost of raising kids -- won't be around after you retire.

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Pre-retirement income: How much you'll need in retirement