Pro and Cons of E85
the stoichiometric air-fuel ratio. For the most common fuels, this, however, is not necessary because the ratios are known:
Natural gas: 17.2
Gasoline: 14.7
Propane: 15.5
Ethanol: 9
Methanol: 6.4
Hydrogen: 34
Diesel: 14.6
from:
http://www.brighthubengineering.com/machine-design/15235-the-stoichiometric-air-fuel-ratio/
E85 would be .85*9 + .15*14.7 = 9.855
For the same amount of boost enrichment (I'm not sure you need the same)
11.5/14.7 * 9.855 = 7.71
With a wideband O2 sensor, either use lambda mode or just pretend you're running gasoline (the 7.71 will show as 11.5)