Oh yes D.P.Davies, first published 1967 and as you so rightly point out, somewhat obsolete. The 55% min. N1 on the JT9 equipped 747 Classics was a Boeing recommendation. Sure you could play with the thrust levers for speed control but idling the outers would most probably run the inners at more than 55% N1 so why not follow Boeing's advice. As I said, we only did flapless approach practice in the Boeing simulator during initial conversion in 1979 and that's the way we were taught to carry it out. Also modern engines have a faster spool up time these days so maybe Boeing philosophy has also changed?