You did'nt mention wether you reckon your friend is too short or too long in his estimation, from what i can find
4650 feet( .88 mile / 1.4 km - 64 feet in 1:72 ! ) is the full run and has a Lanc 50 ft off the ground, (tho i dont know with what load that figure is based on)The 'up-keep' bouncing bomb was fairly light at 9,250 lb (4195 kg) and only ever used on that night .
Heavy single load 12,000 lb tallboys or 22,000 lb grand slams required 'specials' which were lightened by having one or two of the three gun turrets removed. 617 Sqd apparently had problems at Losimouth with runway length and fitted more powerful merlins to cope
Maybe Dai can back -up our posts when he gets back to you,
and settle the questions