I was researching this a while back since I will be tuning a car this winter. I found a few articles that discussed the fact that O2 meters are reading a Lambda value and converting it to AFR on your readout. Long story short, you can just leave the device set to read AFR for gasoline and tune E85 using the same values as you would for gasoline.

Here is one article discussing this:
Lamdba_E85_discussion