Building up a reasonable nest egg for your golden years is a difficult enterprise, and one thing that makes it even harder is that it's not so simple to guess how big a "reasonable nest egg" for you is, or even what your living expenses will be. Among the more frequent errors people make when they sketch out their plans is to assume that their costs will drop significantly. No more commuting, no more work clothes, no more buying five lunches a week from the takeout joint... that has to add up to a pretty penny, right?

Sorry, says personal-finance writer Maurie Backman -- whom Motley Fool Answers hosts Alison Southwick and Robert Brokamp tapped to dispel some retirement myths -- that's not going to save you as much as you think.

A full transcript follows the video.

This video was recorded on Nov. 13, 2018.

Alison Southwick: Let's move on to the second myth. Your expenses will go down in retirement.

Maurie Backman: Right. So a lot of people think, "OK, I'm going to retire, and instead of spending what I'm spending today, my expenses are going to get cut in half." And in fact I have a relative who's close to retirement who said this to me recently. He said, "Yeah, I think I'll probably spend about half as much in retirement as I am now."

And I said, "Why?"

He said, "I won't be commuting. I won't be paying for dry cleaning."

I said, "Well, what are you paying now to commute to work and to dry clean your suits? A couple of hundred bucks a month?"

He was like, "Yeah."

I said, "Does that represent 50% of your spending?"

And he's like, "Oh, no."

The thing about retirement is that, yes, you'll kill a couple of costs. You'll kill your commuting costs and maybe some other incidentals that we all incur by having an office job to go to. I wouldn't know something about that. You guys who actually report to an office every day probably know a little more than I do.

But generally speaking, I like to tell people that you'll probably need about 70%-80% of your former paycheck to cover your expenses in retirement, because most of them are probably going to stay the same. You might even have some that go up, like healthcare or entertainment, because when you think about it, you're not working. You don't have a job to go to. You're not going to sit in your house all day.

So that's the rule that I like to follow. The only thing that's really going to go away, the only major expense, is your retirement plan contribution because obviously you're not saving for retirement when you're in retirement. But when you think about the things that you're spending money on right now, there's really no reason to think that they're going to drastically drop just because you're a little bit older and not reporting into an office.

Southwick: Unless you're going to make major life decisions like downsizing or moving someplace cheaper.

Backman: And I've heard people say, "Well, what about your mortgage because you might pay it off in time for retirement?" And my answer to that is you might, but keep in mind that if you're hanging on to an older home, as that home ages, your maintenance and repair costs are going to go up. They might go up enough to offset that mortgage payment, especially if it was on the lower side. And property taxes also have a tendency to go up over time. There's that to consider, too. So don't expect to pay, like, 50% of what you're paying now to live just because you're old.

Southwick: How often do you have the random relative or uncle or friend coming up to you and offering their thoughts on their own retirement and then you have to be like, "Well, actually..." Does it happen a lot?

Backman: Pretty often. And I feel bad because basically my family members call me the "dream dasher." I'm that person who dashes dreams, but I'd rather give someone a reality check than have to help them pay for their nursing home.

Southwick: That's true.

Robert Brokamp: The Wall Street Journal recently published the results of a study from Dan Ariely and Aline Holzwarth who work at the Center for Advanced Hindsight at Duke University.

Southwick: Advanced hindsight.

Brokamp: I'm going to summarize it very basically. They asked people how much of your pre-retirement income are you going to need in retirement and most people thought 70%-80%. And then when they asked another group to actually break it out into specific categories, some groups had as much 130% because, as one quote in the article said, working is a very cheap activity. And once you stop working you start doing all these things that you've always wanted to do and it turned out you spend much more, at least in the first five to 10 years of retirement.

So the bottom line is you have to figure out what your situation is and what you're really planning to do in retirement before you just rely on a rule of thumb.