This was a discussion many, many years ago.
Yep a few Blue Moons it was....
I posted a link to a PDF file I scanned from a friend's B17 Gunner Training Manual he gave me for fixing his A/C, many years ago...he had some awesome stories of being a tail Gunner in the B17...
Anyhow, that file/link should still work since it is on the Trainer's FTP site...
Been years since I last looked at it but if memory serves me right, the manual calls for opening fire when enemy gets to within 1,200 to 1,000 out..