If the OGE hover weight is determined from the Flight Manual OGE WAT curve and then reduced by 5%, operating at the resulting weight will be the equivalent of operating with a 5% thrust (weight) margin for HOGE. This is likely the most accurate way to make the determination because the Flight Manual OGE WAT curve takes into account the effect of air temperature on both turbine engine power available and rotor power required.
It is true that -40°C and 9,000 ft PA produces the same density altitude (4000 ft) as +40°C and 1,000 ft, and that the engine power available is substantially different when operating at those two conditions. It is also true that the rotor power required will be different when operating at those two conditions due to the different blade tip Mach numbers (compressibility effect).
I note that the Flight Manuals of several modern helicopters (I think the Eurocopter Dauphin Series and the Sikorsky S-76 Series) provide power assurance procedures that quantify above specification engine performance and WAT curves formatted to take performance credit for those engines that produce above specification power.