I don’t know if I agree with the last part that says that your equity position should always be less than 50%. However, the first part seems to offer a good rule of thumb when it comes to investing in a target date retirement fund.
Let’s say you have the Vanguard Target Retirement 2050 Fund (VFIFX) and it currently contains 90% stocks. Using this rule of thumb would mean that a possible one-year loss for such a fund is 45%. You should ask yourself – can you handle a 45% drop in the value of your retirement assets, even if you have 40 years before you need it? The good thing about living through 2008/2009 is that you probably have a better idea of the truth. If you’re going to run for cover in cash, only to buy back in later (like now) when prices are 50% higher, then that’s something to avoid.
One thing that I recommend to my more conservative friends who still want a simple investment is to simply buy a different “date”. For example, if you could purchase the Vanguard 2025 Fund (VTHRX) which has 75% in stocks. Who cares if the label is 2025. Meanwhile, I encourage them to continue to learn more about investing so that the can understand the risks trade-offs better and adjust their tolerances accordingly (up or down).