Track your net operating income after all expenses, including capital improvements, for several years. Track how much the rents increase annually, how much you spend on taxes, insurance, HOA's, repairs and maintenance, and capital improvements and how much these increase. Figure out how long the properties are vacant, and your collection loss. Include a management expense, even if you manage the units yourself, as you may not in the future.
Once you have a handle on this, you can project some averages into the future. You cannot predict when you will have to make capital improvements, when rents will go up or down, and what your vacancy will be. Estimate income and expense, and grow them at a reasonable rate.
The net income, which is smoothed, is what you use in your calculations of what you need from other sources.
If you have mortgages, subtract the principal and interest from the net income for those years you will have the mortgages. You may need more income from other sources in the early years if you retire before the mortgages are paid. For example, you may have $20,000 a year in debt service for 5 years after you retire, and $10,000 for another five years. You may need some padding early on to cover that.
How I would handle Social Security would depend on your age and how many years you estimate you will pay in. A 25 percent haircut in benefits in 2034 is a common assumption if you are under 50.
Remember, everything is an estimate. Precision to the extent reasonable is useful, but your precision may exceed your accuracy when you predict the future.
