Google, "NE555 calculator", second result:
That does what you're wanting. It assumes a practical value for one of the
components and calculates the other values from the freq and DC input.
I suppose the more intelligent way to use that calculator is to enter
duty cycles that are arbitarily close to and above but not exactly equal
50%. Of course, you wouldn't know to do this if all you've ever done is
use calculators. Entering 50.3% , 1KHz, and C=0.001u, values obtained
with some fairly rough insight into 555 operation, returns RA=8.64K and
RB=716K- not std values according to my other formula derived
mysteriously from additional applications insight, almost as good as,
but a lot less fun than, the manual method.