I ran E85 from 2008 to 2012 when I switched to methanol (cost).
IMO, a couple percent difference is not a big deal as long as you are rich, not lean. I used an oxygen sensor for tuning on my car, but took it out for normal racing.
I would rather use 93 octane gasoline to bring down the 88% stuff to 85% and use 85% as your baseline. (or just leave it 88%).
Now if you had 75%, I would add ethanol or E98 to bring it up to the baseline (85%).