Granard,
Frustrating isn't it when no one answers your question, so here goes,
Threshold distance (ETOPS) is ;
Maximum distance from an adequate aerodrome for twin engine aeroplane without ETOPS approval. Based on ;
1) Diversion after engine failure
2) Take off at MTOW and gross weight after one hour of flight time
3) Mmo/Vmo speed schedule
4) zero wind
5) ISA conditions
Hope that clears it up