Per NIST[1], the value is 0.7204% +/- 0.0006%, with the uncertainty representing one standard deviation.
[1] https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl...
I think it would've been good enough for the miners too, if not for the fact that nuclear arms control treaties require every gram of U-235 to be accounted for. When they were digging it out of the ground and found it was less enriched than it should've been, this needed to be explained. It has always fascinated me to think that this natural phenomenon could and probably would have remained unknown forever if not for these treaties and agreements.
Heh. The garbage web software developer me would have just called it good enough
Would be really interesting to know what the error bars on those figures look like