TSP withdrawal

Q. I am under the CSRS retirement system. I have 32 years of federal service. I am 56 and plan on retiring at 60. I contribute to the TSP, $17,500 per year and an additional $2,600 catch-up to the TSP per year. I am a GS-13/7. My high-3 would most likely be at this pay grade. How can I calculate how much my monthly annuity would be upon retirement so my financial adviser will have a better understanding of my situation upon retirement? I know about the ballpark estimates, but I would
like something a bit more concrete. When do I need to start withdrawing from my TSP fund upon retirement? At what age?
A. You must start withdrawing from your TSP by April 1 of the year following the year in which you reach age 70-1/2 or the year after the year you retire, whichever is later.