BOAC:
Maybe I can help, maybe I can't.
In Laker, CAA rules required a 5% contingency to be carried for the part of the route where no enroute alternates existed. So, for a DC-10 flying from LGW to JFK we were required, in effect, to carry 5% of the fuel burn between SNN and YQX which amounted to about 2,000 lbs.
When I flew identical DC-10s for a US Flag carrier (Part 121), we were required to carry 10% of the entire fuel burn from LGW to JFK.
This simply could not be done whilst carrying a decent (identical) payload so we used the Reclear Flightplan System. The ATC flight plan would be filed from LGW to YQX with full reserves. We also carried a flight plan for the entire trip to JFK.
So, approaching YQX, if the fuel on board equalled the fuel required to make the non-stop flight (IE: you now only needed 10% of the burn from YQX to JFK then the flight could continue after being recleared by the despatcher.
If not, then we would reclear to BGR and, ultimately, to BOS by which time all that was required was 10% of the burn from BOS to JFK.
It was an unwieldy system but it worked. I never failed to make the direct flight.
Incidentally, if it was a Supplemental Flight (Part 135) then the captain could make the decision to reclear without consulting the despatcher.