The TSP will take out 20%. But, your actual taxes may be more or less than 20%. It depends upon your marginal tax bracket in the year of the withdrawal.
Many federal employees, especially if their spouse also is employed and file jointly with the spouse, will be in a 25% marginal tax bracket. So, the mandatory 20% withholding will not be enough to cover the taxes owed by at least 5%. As other posters have noted, state taxes may have to be considered. These could be issues even for individuals who are over 59.5 years of age and not subject to the 10% penalty.
On the other hand, many posters here have had significant periods of LWOP and/or do not have a working spouse (or any spouse at all). They may well be in a lower than usual tax bracket in the year of the withdrawal and, if not subject to that 10% penalty, might wind up getting a Federal tax refund when they file their taxes the following year. Also, some states, like TX and NV, do not have income taxes.
The point is to get a handle on what your particular tax situation might be when you make the withdrawal. I believe that you can specify a larger withholding for taxes, if you think it might apply. If you are pretty sure that you are going to get a refund, then you will want to file your taxes as early as possible in the year following the withdrawal. That way you receive your refund sooner rather than later.