For comparison, the U.S. specification MIL-C-46281F of the 7.62 mm M62 tracer bullet requires “full luminosity” at a point not more than 125 yards from the muzzle.
For the older cal. 30 M25 the figure had been 100 yards (MIL-C-1317E).
Peter Labbett in “The development of 7.62mm x 51 Ammunition in Britain 1953-1997” does not mention the dark ignition distance of the L5A1.
The NATO AOP-2310 [requirements have been moved from STANAG 2310 to this “Allied Ordnance Publication”] requires:
“dim or invisible from the muzzle to at least 13 metres; shall be visible by 140 metres and shall maintain continuous visibility between 140 metres and a minimum of 775 metres.”
According to Labbett, these figures applied to the L5A3, approved in May 1962.
Edit: The U.S. specifications can be found on quicksearch.dla.mil
Enter only the numeric part in the document ID field. Don’t panic, only the newer versions are restricted, the older can be downloaded as PDF.
The NATO website nato.int has a STANAG page.
Labbett’s book was self-published in 1998 and I do not know its availability in libraries.