Since octane numbers are just a way to keep track of the temperature at which gas will or will not pre-ignite AND compression is just one of a few things that determine pre-ignition, you have to make up your own formula. Ambient temperatures, seasonal fuel mix, engine cooling abilitys, loads. speed, terrain, and GVW can all affect pre-ignition.
Yes...Oxy-fuel is spreading thru the country and is "crappy", low mileage, low power gas and will affect engine power and cooling ability. It will lead to early pre-ignition.
I'd try to talk to people who are already using that octane and try to match my driving conditions and compression to theirs.