Dear Liz: None of the Web-based tools I’ve seen really get at the heart of the problem of how much I really need in retirement. For example, if I am diligent and save 20% of my income (I earn over $150,000), why would I need to replace 95% or even 80% of my income to maintain my standard of living in retirement? If I subtract the 20% going to savings, another 10% for the costs of working (clothes, lunches, gas) and reduce my income tax 5%, shouldn’t I be living the same lifestyle at 65% of my current income? Now, if I have a pension that will replace 10% of my pay, and if Social Security benefits for my spouse and me replace 30%, don’t my investments have to produce only the remaining 25%? Or am I missing something?

Answer: The further you are from retirement, the harder it can be to predict how much you’ll need when you get there.

Financial planners often use an income replacement rate of 70% to 80% as a starting point. It’s just that, though. Planners will tell you some of their clients’ spending actually increases in the early years of retirement as they travel and indulge in other expensive hobbies. Those who are frugal or used to living well below their means are often able to retire comfortably with a much lower income replacement rate.

A big wild card is the cost of medical and nursing care in your later years. The U.S. Bureau of Labor Statistics’ Consumer Expenditure Survey shows average overall spending tends to drop after retirement and continues to decline as people age. Serious illness or a nursing home bill can cause spending to surge late in life, however, leading to a U-shaped spending pattern for many.

Taxes also are hard to predict. While most people drop into a lower tax bracket once they stop working, those with substantial retirement incomes and investments may not. Tax rates themselves could rise in the future, even if your income doesn’t.

Social Security benefits may change, as well. Although it’s highly unlikely the program will disappear, some proposals for changing Social Security reduce checks for higher earners.

Once you’re within a decade or so of retirement, you should have a better handle on what you’ll spend once you quit work. Before that point, err on the side of caution. Assuming a higher income replacement rate gives you wiggle room once you’ve retired — or the option to retire earlier if it turns out you need less.