The first step to effectively using your 401(k) is to assess your retirement needs. Thinkstock By the editors of Kiplinger's Personal Finance Updated January 2015 How do you come up with the right amount to divert to your 401(k) plan?Financial planners have traditionally estimated that retirees need 80% or more of preretirement income to maintain their standard of living, but individual situations vary greatly. Paying off a mortgage before retirement can substantially reduce need for income, while paying for health insurance can radically inflate it. See Also: The Basics Special Report H. Jon Miller, vice-president of the personal financial-planning division for the Northern Trust Co., offers a shorthand formula for figuring out how big a nest egg you need to finance your retirement. Starting at age 65, figure you can spend 4% of your retirement portfolio the first year in retirement and increase that dollar amount to keep pace with inflation in subsequent years. Under that conservative formula, your money should last through a 30-year retirement. But if you encounter a bear market during your early years in retirement, you might want to skip the annual inflation adjustments until the market recovers. Withdrawing too much from a shrinking nest egg can have devastating effects. Here's the math: Assume you need $75,000 a year and that social security and a company pension will provide $30,000 of it. According to Miller's formula, if you divide your unmet need ($45,000) by 0.04, you need to start retirement with a portfolio worth just over $1.1 million. Another way of thinking of this is multiply how much annual income you need to draw from your savings and multiply by 25. Advertisement Taking the time to figure out how much you will need after your paycheck stops could mean the difference between a comfortable retirement or working a lot longer than you had planned. Once you go through the process to figure out how much money you will need in retirement, then you can figure out how much of your salary you should be deferring in your 401(k) plan and how you should be investing that money to reach your goal. For example, if you are having a hard time deferring 5% of your income and yet you need to set aside more to reach your goal with your current investments, maybe you should adjust your investment strategy to be more aggressive, assuming you have more than five years before you retire and you are comfortable with the added risk. Diversify your investments Regardless of what's offered inside your plan, your objective is to earn a reasonable return over a long period while limiting risk. Market cycles tend to favor one type of investment at the expense of other investments. But cycles change, and because no one knows when that will happen, it's wise to spread your investments over several categories. If your 401(k) plan is your only major retirement investment, distribute your money among several mutual funds. But if you have investments outside the company plan, make sure your overall retirement stash reflects a good asset mix, even if it means your 401(k) plan is heavily concentrated in one or two areas. Advertisement Two-income couples should adopt a similar strategy. You should look at the two 401(k)s as part of a whole, allocating your contributions to the best funds each of your plans has to offer. Of course, you have to work within the investment options available to you. Typical plans offer about 18 investments, including stock and bond mutual funds; balanced funds, which invest in both; "target date" or asset-allocation funds, which include a mix of investments based on your age and how much risk you are willing to take; and fixed-interest investments, such as money-market accounts. About 15% of plans now allow participants to "self-direct" their account by buying individual stocks and bonds.