One-time withdrawal

Q. I retire in less than a month under FERS. As soon as I retire, I plan to use the option to withdraw my TSP in monthly increments that I decide every year (I am not using the life expectancy or annuity options). I also want to do a partial withdrawal (not much of it, just enough so I have a good savings account to fall back on). I know I can only do it once. I assumed, or read, that the one time had to be immediately upon separation. I do not really need the partial-withdraw funds right away, but do not want to miss the opportunity to do it. Can you do your once-in-a-lifetime partial withdraw any time after you retire?
A. Once you start the monthly payments, your only options are to change the amount of the monthly payments once per year or to take a final, full withdrawal and close your account. If you want the partial lump-sum withdrawal, you’ll have to do it before you start monthly payments.