Just use the octane number as required by your car.
The issue with ethanol isn’t directly related to octane. It’s that it can damage rubber parts in the fuel line. Particularly in cars and motorcycles that are several decades or more old. And more than ~15% ethanol would not be advisable for most modern cars as it has different properties than gasoline and your car likely would not run well on it unless you have a bifuel system.
Just use the octane number as required by your car.
The issue with ethanol isn’t directly related to octane. It’s that it can damage rubber parts in the fuel line. Particularly in cars and motorcycles that are several decades or more old. And more than ~15% ethanol would not be advisable for most modern cars as it has different properties than gasoline and your car likely would not run well on it unless you have a bifuel system.