maxalt,
Your ETOPS threshold is based on the driftdown strategy chosen by the operator - Airbus offer two or three from memory. Minimum rate of descent is rarely the best for pushing out the threshold - there is some optimum below Vmo that maximises TAS vs height loss and thus maximises distance over the ground for the interval (60 minutes in your case) under consideration.
It isn't cheating per se as the whole business is firstly a desktop planning exercise, the aim of which is to maximise the distance travelled or, conversley, the distance from an adequate aerodrome. That planning is usually ISA nil wind, again if my memory serves me correctly.
Your published OEI TAS is your end state, which you approach somewhat asymptotically from above in both altitude and airspeed.