I put more E85 than ever into my FEH this tank.
Not sure of the exact proportions due to the mystery of what is really in the E85...( and E10 for that matter ) but I'm using 4 gal E85 to ~11 gallons of E10 and I'm getting the same MPG as just E10, +/- 1.9 %. ( +/- 0.5 MPG )