I would not begin to challenge Lem's figures, Mutt, but I suspect from the way the 'q' was worded that (as LEM says) it was about the figure for the take-off alternate calculation requirement in JAROPS which, as I said, was 330nm in BA for the Classic and typically 340nm for a NG.
For this 'reality'/ETOPS considerations do NOT come into it
I deleted my first post which was getting well into complicated territory, and would agree that the 'beast' IS just after the distance for the take-off alternate.
This is based (in JAROPS) on the AFM declared 'single-engined cruise speed' and is used purely for PLANNING the alternate. It has very little to do with how far you would actually go in one hour, which is probably close to Lem's figure. I.E. In reality if you flew for 1 hour after take-off, engine out and only got 280 miles in that time towards your nominated (and 'legal') t/off alternate 330 nm away, you would obviously keep going until you get there.