I came across an interesting and informative article the other day - "10 Financial Myths Debunked". An especially relevant myth is assuming long-term average stock market rates of return of 8 percent as are certain rules of thumb when planning for retirement.
Point 4 is relevant because most people are way too optimistic when it comes to calculating potential rates of return during retirement planning. From a young age, it pays to err on the side of caution and assume anywhere from a 5-7% long-term rate of return on your portfolio. If you have the time, use a financial calculator to plug in your current investments, time horizon and potential rates of return from 5-7% and see where you end up during the year you plan to retire. By assuming a smaller potential return, you will be able to set up the best possible plan to reach your goals and ensure that your investments don't overpromise and underdeliver.
It's also worth noting that "rules of thumb" in the investment world are rarely worth the paper they're printed on because each person is different and the world is not static. Since the world we live in today will be drastically different than the world we will live in 40 years from now, it's not worth prescribing to certain rules that simply can't change with the times. For example, it's impossible to tell what type of health we will be in, what our family situation will be like and how the world will really be so far into the future. Thus, we should plan our investments out based on our own goals and risk tolerances, while avoiding "catch all" rules of thumb that don't accomplish much.