The ‘visible horizon’ is much further away if the observer and the object are both at 35,000ft, of course. If my calculation is correct, I get about 700 miles for the distance the two could see each other if the air were clear enough.
IIRC, they were ~1400 miles apart. Something like 7 degrees of latitude and 19 degrees of longitude. I'm trying to find the translated article that includes the Air Comet position, but I can't seem to locate it.