What matters more than knowing how much of your savings you can safely spend each month? For a federal retiree, not much.
If you don’t know the answer, you run the risk of spending too much, running out of money and destroying your financial security in retirement.
On the other hand, not knowing what amount is safe to spend may lead you to live too frugally depriving yourself of the standard of living you should rightly enjoy. You retired for a reason, right?