Folks,
EROPS route designation is a desktop planning exercise conducted at nil wind ISA and a distance determined by how far a particular aeroplane will go when descending at MCT/P from maximum CRZ altitude at a particular reference weight to its OEI CRZ altitude. That distance will normally be further than that achieved at steady state OEI CRZ, simply because the descent strategy may involve MMO/VMO speeds.
Once the required "adequate aerodromes" are identified within the EROPS Operating Area, the prevailing enroute winds on the day of operations are irrelevant for other than fuel planning. However, for the flight to proceed, the terminal conditions at each adequate aerodrome must be such that those aerodromes are "suitable aerodromes" (in ICAO/FAA/JAA speak, not the Oz mish-mash definitions of CAO 20.7.1B/Part 82) during the relevant EROPS diversion windows.
Sensibly, non-EROPS flight planning should be conducted the same way. However, in Oz the various regulators have never understood the issue or provided relevant direction to their inspectors or the industry. I do not hold much hope that you will be able to convince your local FOI about what is either sensible or practical.
Stay Alive,