Thank you Shortround!
Unfortunately, I've not the book you mentioned. I note, though, it gives hint of an official document. Also, I think I missed last statement by Greyman, about "full RAF switch to concentrated pattern", which should mean they all harmonized to 350 yards from February 1940 onward (although I'm skeptic this could be true for all fighters, with all kind of weapons). Would be useful to know his source.
I think it's curious that there's lack of these publications in any form on the internet; I can find plenty of flight manuals about British fighters, but none that I know reports a "gun convergence" section. Maybe there was an unique official guidance, but so far none made it public, seemingly. Could it be still classified? Or maybe I'm too inquisitive?