I never have much faith in the OBC's reading, in the C4 I used to drive, once the fuel light came on at ~40 miles with 2 flashing bars, you could do those 40, then have no flashing bars, then do another 20 before its time to look for a fuel station. Then you put a tenner in, and apparently you'll only get 50 miles out of that.
A friends Pug 406 2.0 diesil was similar, once the fuel light came on, he could drive to work and half way back before it started complaining (~80 miles).