Thanks for sticking with me on this......I just checked the NAT doc 007 which I assumed to be the controlling document. This is where we get our three minutes or more from ....
Natdoc007 6.3.4 “Clearance.
Estimate for Oceanic Entry
After obtaining and reading back the clearance, the pilot should monitor the forward estimate
for oceanic entry, and if this changes by 3 minutes or more, should pass a revised estimate
to ATC. As planned longitudinal spacing by these OACs is based solely on the estimated
times over the oceanic entry fix or boundary, failure to adhere to this ETA amendment
procedure may jeopardise planned separation between aircraft, thus resulting in a
subsequent re-clearance to a less economical track/flight level for the complete crossing.
Any such failure may also penalise following aircraft.”
my company interpretation of this has been 2 mins 59 seconds maximum to avoid missing by three minutes or more.
I know this is your area of expertise (and the Jeps chart backs up your two minutes) but we have conflicting information here (and no pilot is aware of the 30 second tolerance.....which is a good thing because if you tell us there is a tolerance we will too often use it to the max).