I always was taught 1/2 scale deflection having watched it start from full scale deflection. VOR and NDB approaches...once established "on" course. (although I favored the "artillery" technique....that is bracketing the target)

Established on Course.

ICAO defines “established on course” as being within half full-scale deflection for an ILS or VOR/TACAN/RNAV/GPS procedure and within ± 5° of the required bearing for the NDB. Adherence to the ICAO standard will insure you are within protected airspace when conducting an approach. Therefore, do not consider yourself “established on course” until you are within these limits.

Possibly found in ICAO DOC 8168, but not sure.