New retirees frequently rhapsodize about the joys of tossing their alarm clocks into the trash and filling their days with whatever activities they find gratifying. But if they are honest, most new retirees find the financial aspect of the retirement transition to be a little jarring.

Estimating actual cash-flow needs is a tricky business. While retirees are often counseled to estimate that they will spend 75% to 80% of their working incomes in retirement, there can be huge variations in income-replacement rates among retirees—with factors such as pre-retirement income and savings rates serving as key swing factors. Higher-income, higher-saving households may need just 60% (or even less) of their pre-retirement income during retirement, while lower-earning, lower-saving households may need closer to 90%.

Ultimately, it may be difficult to forecast your actual income-replacement needs with a great deal of precision. Even as you attempt to anticipate every in-retirement expense to the penny, unforeseen expenditures such as healthcare costs can buffet spending around on a year-to-year basis. But because anticipated income needs are such a key ingredient in the retirement-income puzzle, it is helpful to come up with as realistic a figure as possible while also being realistic that your own expenditures are apt to vary over time. Here are the key steps to take as you do so:

Step 1: Find a Realistic Baseline
A key starting point when determining an income-replacement ratio is your working income. If you are close to retirement and seek to maintain a standard of living in retirement that is similar to what you had while you were working, using your current salary as a baseline is reasonable. But if you are younger—say, in your 40s—it may be wise to nudge up your baseline income for retirement-planning purposes, because your current income may not be reflective of what you will want to spend when you eventually retire. Not only are you apt to receive cost-of-living adjustments as the years go by, but career gains could also lead to a higher salary over time, which you may want to “replace” in retirement. One study asserts that the average college-educated individual will make a 50% higher inflation-adjusted salary at retirement than he or she did at age 25. Gains in salary over time are less pronounced for people with lower levels of educational attainment.

Step 2: Subtract Your Savings Rate
The next step in arriving at your customized income-replacement rate is to take a look at what percentage of your salary you are saving—or expect to save by the time you retire—and subtract that from your baseline salary amount. One of the reasons higher-income individuals typically have lower income-replacement rates than lower-income people is that the former are able to save a higher percentage of their salaries during their working years; they need less of their salaries to fund basic living expenses. A household saving 20% of its income will see its income-replacement rate drop to 80% right out of the box, even without factoring in any planned lifestyle changes, such as downsizing homes. If you are several years from retirement, it may be that you will kick up your savings rate if your income grows.

Step 3: Subtract Any Tax Reductions
Because they are no longer paying Social Security or Medicare taxes, many people realize tax savings when they retire. Those gained savings tend to be more pronounced for higher-income workers than lower-income ones. More affluent households may see a bigger percentage drop in taxes in retirement than lower-income households because they have greater control over their taxable income now that they are no longer earning a paycheck; the less they pull from their portfolios, the less they are taxed on. Moreover, even as both types of households are drawing income from their portfolios to fund in-retirement living expenses, the higher-income household is apt to be able to have more levers available to keep taxes down—for example, drawing just enough assets from traditional, Roth, and taxable accounts to stay within the lowest tax bracket.

Step 4: Subtract Any Anticipated Housing-Cost Reductions
Housing costs are another line item with the potential to change substantially in retirement. Is your plan to come into retirement without a mortgage, for example? Or perhaps you intend to relocate or downsize in some fashion? Even though the main goal of downsizing may be to add the home-sale proceeds to your retirement kitty, it can have the salutary effect of reducing property taxes and lowering outlays for insurance, utilities, and maintenance. As a senior homeowner, you may also be able to qualify for a reduction in your property taxes, depending on where you live.

Of course, not every household sees a drop in housing costs during retirement; some retirees stay put in their primary residences while also purchasing second homes that actually add to their total housing-related outlays.

Step 5: Factor in Lifestyle Changes
Retirement-planning guides often urge retirees to factor in changes in other expenses, such as commuting, clothes for work, and meals out while on the job or due to busy work schedules. For some households, these expense changes may be minimal, but for others they may be more substantial. Research points to food costs as one of the expense items likely to decline the most in retirement; one paper showed a 5% to 10% drop in food expenditures for households following retirement, while another showed a 6% decline. Not only do retirees have more time to prepare food at home than they did while they were working, the researchers conjectured, but they also have more time to shop for grocery bargains. As with housing costs, lifestyle-related outlays are not guaranteed to decline in retirement, so do not assume a reduction in yours without crunching the numbers. If a heavy travel schedule or an expensive hobby is on your retirement to-do list, you might see any cost reductions on line items like food offset by increased expenditures elsewhere.

Step 6: Add Higher Healthcare Costs
Thus far, we have focused on ways that retirees might expect to see their expenses drop in retirement. But there is one major area where they are likely to increase, and that is in the realm of healthcare. A recent study showed that the average out-of-pocket healthcare outlay for a retired couple was nearly $300,000, and that figure does not even include long-term care expenditures.

Healthcare expenditures are a bigger share of the consumption basket for elderly households in the Bureau of Labor Statistics’ Consumer Price Index calculations; on average, older adults devote almost twice as much of their outlays to healthcare expenditures than is the case for the general population. Increases in healthcare costs at large may be a key reason that the CPI-E has tended to be about 5% higher than the general inflation rate.

Not only have healthcare costs outstripped the general inflation rate, but they also tend to trend up through retirees’ own life cycles. Higher healthcare costs later in life are the key reason for the retirement spending “smile”. That is the tendency for household expenses to be on the high side just after retirement (when spending on travel and leisure is apt to be high), dip in mid retirement, then head back up toward the end of life as healthcare costs increase. If you are someone who is going without long-term care insurance, in particular, recognize that your household’s total healthcare-related outlay could spike dramatically toward the end of your or your partner’s life.

Step 7: Add a Fudge Factor
Working through each of these line items may get you closer to your actual income-replacement rate rather than relying on rules of thumb such as 75% or 80% for income replacement. At the same time, it is worthwhile to approach the exercise with the knowledge that there is much about your future spending that you cannot foretell. Long-term care costs are the biggest wild card for people who do not have long-term care insurance or for those who have policies that are capped at specific benefits. Many seniors have also been called upon to help their adult children or their families, unexpectedly increasing their financial outlays in retirement. Homeowners, too, can incur costly and unexpected repair bills at random times. All of these factors can send your expenditures out of line with what you thought they would be. The potential for those unanticipated expenses argues for nudging your own income-replacement rate a bit higher to allow for some wiggle room in your planning.