Unfortunately that's not how it works.
Bumping the ethanol content in petrol up will keep the octane the same because they will offset the increased octane from the Ethanol with lower octane petrol; that's the whole point of this I think; they're not simple mixing the existing petrol with pure ethanol.
The names of 93 octane or 91 octane implies its resistance to knock, but the names E50 or E85 imply the % of ethanol contained. IIRC E85's octane rating is somewhere north of 100 and E50 is in the mid-high 90s.
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