You can't accurately measure the octane of ethanol. The octane engine used for measuring don't account for the charge cooling effect. This really doesn't matter, since as mention above, ethanol is ethanol.
Shoot for a specific level of ethanol. Tuners call their tunes "E30", not "100 octane".
If you run E30 fuel on a stock tune, the oxygen sensor will "see" the oxygen in the ethanol as missing fuel. The ECM will add more fuel to make up for it. E30 has 6% less energy than E10. If you monitor the long term fuel trims (LTFT), you will see they will increase the fuel volume by 6%. If the LTFT go too far out of whack (~25%), the Check Engine will come on.
An E30 will adjust the Air Fuel Ratio to bring the LTFT back to zero. It will then take advantage of the higher octane by running more timing and/or more boost.
The best thing anyone can do is monitor their knock retard. Keep it at zero and keep the octane adjust ratio at 1. This allows maximum timing and maximum boost. If you consistently see knock, ask your tuner to dial back you tune a little.
Ethanol is cheaper than gasoline. The only reason to lower ethanol content is for cold starts in the winter. This means nearly every station will be at maximum ethanol during the summer.
And one last bit of trivia. It was originally called Ed85 for 85% denatured ethanol. At best, it's 83% pure ethanol. It's never been 85%.
Oh, and those little handheld test tube testers, they are very inaccurate. But don't worry, if you are monitoring knock, you don't need to be spot one with the E30. 25-35% is close enough.
