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In Canada, the United States, Brazil, and some other countries, the octane number you'll see at the pump is the average of the RON and the MON numbers used elsewhere. It's called the Anti-Knock Index (AKI), and it's often posted on pumps as (R+M)/2. That's the number you need to know. The actual percentage of ethanol to gasoline isn't relevant. You just want to know how resistant the final fuel blend is to detonation. 112 (racing gas) is better than 102. 102 (lesser quality racing gas) is better than 92 pump gas, and so on.

The percentage of ethanol to gasoline is important when it comes to your air-fuel ratio. Ethanol contains less chemical energy than does gasoline, so a "gasohol blend" will tend to run lean (hotter in the combustion chamber). It's possible to lean out your fuel mixture and burn a head gasket or a piston without detonating the engine when using E-85.