When you plan for retirement, some simple numbers can help you develop an investment strategy. Start by determining how much you spend each year: that’s gross income, minus taxes and investments and exceptional outlays. Your current annual spending should be your target for retirement spending.

Rule of Twenty. Figure how much you’ll need from your portfolio each year. If your goal is to spend $80,000 a year in retirement and you expect $50,000 from your retirement benefits, you’ll need $30,000 from your portfolio. Multiply that $30,000 by 20 to get $600,000: your target for how much to accumulate in your investment portfolio.

Rule of Six. Put six times your projected outlays into fixed income assets such as bonds and bond funds. If you want to take $30,000 from your portfolio each year, multiply by six to get $180,000: the amount you should hold in fixed income. That will provide spending money for six years, regardless of what happens in the stock market. The balance ($420,000 out of $600,000, in this example) should be in a mix of stocks and stock funds, for long-term growth.

Each year, move money from stocks to fixed income to maintain your six-year allocation. Sell losing stocks and stock funds when you can, for capital losses. Those losses can shelter the taxes you’ll owe when you sell stocks at a gain.

Rule of Three. Typically, you’ll increase your spending at 3 percent a year, to keep up with inflation. However, you may want to keep spending constant in years when stocks fall sharply.