Ann Pircio Pardes, 62, director of early childhood education at a religious school in Westchester County, New York, wants to retire in about five years. But given a family history of lon­gev­ity, she’s afraid she’ll outlive her money, so she wants certainty: an accurate forecast of her retirement income. As a divorced single woman, she also worries about her long-term care and the cost of insuring it. “I’m not sure my future old-lady self can afford it,” she told me.

Creating a useful retirement plan requires a lot of detail—things like your future monthly expenses and future investment returns. But that involves guesswork, and unexpected events can derail even the best guesses.

Financial adviser Margarita Perry of RBC Wealth Management was up to the challenge. First, she inventoried Pardes’ assets and debts and got a picture of her cash flow. Pardes’ assets are chiefly $300,000 in a 403(b) retirement account, an $88,000 annuity and about $150,000 in cash. Her only debt is her $350,000 mortgage.

As for cash flow, Pardes spends about $4,600 of her $6,500 take-home pay each month. She saves most of the rest, partly for a travel fund; her “someday” dream is an African safari. Her monthly Social Security benefit would be about $3,600 if she claims at 67 or over $4,000 if she waits until 70.

After Perry studied Pardes’ investments, she had some questions:

• Would Pardes consider selling and renting instead of continuing to pay her mortgage? Yes.

• When might she need a new car? Not anytime soon. She plans to drive her 2018 Honda into the ground.

• Would she mind part-time work in retirement? She’d like it. She could easily imagine teaching.

• What does she worry about? Longevity. Pardes’ parents both lived into their 90s, though her mother had a massive stroke in her 70s.

It’s impossible to predict the path of your investments, health and personal goals over 30 years. So planners use computers to explore outcomes under different scenarios: What happens if you sell the house? Delay Social Security? Work part-time? Their software calculates how often your plan will succeed—for Pardes, success is to not run out of money—when run through numerous possible investment returns. Advisers differ on what rate of success to aim for; Perry regards success in at least 75 percent of the hypotheticals as a win.

One of the four scenarios Perry investigated had Pardes retiring and taking Social Security at 67, working part-time until 70 and selling her home. Without long-term care insurance, however, she had only a 56 percent chance of success if she needed care. The final scenario Perry ran also had Pardes retiring at 67, working part-time and selling the house, but delaying Social Security until 70 and buying a long-term care policy. Here, she had a 73 percent chance of success—close enough to 75 percent to make Perry favor this approach.

In all scenarios, Perry increased the stocks in Pardes’ portfolio to about 60 percent and added more to her travel budget. “You’re young!” Perry said.

Finally, Perry said long-term care insurance wasn’t out of reach. She thought Pardes’ good health enabled her to buy a hybrid life/long-term care policy; a suitable one might cost $12,000 a year over the next 10 years.

Newly confident that she can retire in five years if she wants, Pardes is taking action. She’s gotten two different quotes for long-term care insurance. She has decided she’ll sell her house as soon as three years from now. And wanting more than a one-time financial checkup, she has hired a fee-only financial adviser—a woman who doesn’t profit from any investments she recommends. Now that Pardes has a plan, she says, “I need someone to help keep me on track.”