BOSTON ( TheStreet) -- There shouldn't be any harm in finding ways to simplify the complexities of retirement planning.Over the years, much advice has drawn upon "magic numbers" touted as simple guidelines and goalposts for investors. Those cookie-cutter bits of wisdom, however, hardly reflect the changing face of retirement, an evolution charged up by the millions of Baby Boomers who are just now reaching retirement age. Some may be downright dangerous to follow. Here are some common measures and how they stack up. The magic number: 70% What it means: Plan to spend 70% of your current income in retirement. For example, if your pre-retirement income was $100,000 a year, spend $70,000 in your golden years. What's wrong with it: People are living longer than ever. (The life expectancy in the U.S. is 78.4.) With a company pension, you may be set for life, no matter how long you live. With direct-benefit plans giving way to employee-managed 401(k)'s and IRAs, retirement savings has become finite and the risk of running out of money as you grow older becomes very real. That is not to say you should live out your remaining days as a penny-pincher. But it does mean that you need to individualize your game plan. Do you come from a family with some members who have lived into their 90s? Do you have an illness that may someday require long-term care? Those and other personal factors need to be among the many things to think about as you assess your post-retirement spending habits and investment strategies. The magic number: 4% What it means: This is another spending guideline. It suggests that a retiree spend an inflation-adjusted 4% of his or her total retirement assets each year, keeping the balance invested with a mix of stocks and bonds. What's wrong with it: No less an authority than Nobel laureate William Sharpe, professor of finance, emeritus, at the Stanford Graduate School of Business has written extensively about this "rule" and why it can ultimately be harmful. The rigidity of the spending plan is among its problems. If a portfolio underperforms, staying the course is a clear path to running out of money. When returns are better than expected, there is an unspent surplus.