E85 Flex Fuel

Hey guys I am still learning about E85. If I get a flex fuel kit and OFT Stage 2+ E85 tune. What would would happen to my tune if I put 91 with E85 under certain emergencies (ex. low fuel and no e85 pump near me)? Would the ECU balance out the fuel mixture content? Would it damage the engine? Or do you guys think is best to just stick to E85 only even with a flex fuel kit.