I think the "established" point has two elements.
One is for ATC sequencing, the other for safe descent:
It is not unknown to called established as the ILS pointer moves across, because if one has armed ILS capture, modern airliners will very rarely not capture, assuming correct speed and config for that phase of flight. And it helps ATC sequencing if they know you are within a gnat's crotchet of being there rather than them having to move planes around in the sequence. Would ATC agree?. I would say "becoming established" to indicate this. Likewise, if we were intercepting an NDB track, I might call established if I could see that the trend was reliable - we might be +- 6° but by the time I had finished the radio call, we would be within +- 5° for example.
The second point is being actually legally established for the purposes of descent, which must obviously be within the published limits, otherwise, CFIT or terrain issues could occur.
I am sure that most TREs would use their sensible discretion on this.