It heavily depends on your tune, but unless you really lean it out I’d say 15% is a very, very generous.
30-35% decrease is about what you should be getting if you do nothing but scale your VE. The O2 sensors will force proper mixture in closed loop. Lambda is lambda as far as the sensor is concerned. 9.765 for E85 = 14.7 for gasoline = 14.13 for E10 = lambda 1.0
I’ve run E85 since it became readily available, so I have a little experience.