What percentage will you need to earn from your investments each year to meet your retirement goals? That number is your required rate of return.
This website has a calculator that allows you to input different rates of return to calculate the future value of your TSP account balance and contributions. However, the challenge arises in determining whether your required rate of return is realistic given your time horizon, your willingness and ability to take risk, prevailing market conditions, and other constraints that may be specific to your situation.
If, for example, you calculate that, to meet your goals, you'll need a 15% annual rate of return, you will likely fall far short. You'll need to go back to the drawing board and either increase your savings or reduce your retirement income expectations. Most experts agree that it is unrealistic to expect such high rates of return, especially over the long term.
After a review of long-term historical rates of return, you may conclude that lowering your required rate is more realistic. The next step is to consider which combination of available investment vehicles will generate that rate. You'll also have to think about the amount of risk those investments carry. Perhaps 7% is a realistic goal, but, if you've invested all of your money in the G Fund, the likelihood of earning a rate that high is slim.