I fill 3/4 with "e85" and start the car and look at what the ethanol content analyzer is saying. My goal is E60-65 (tuned at ~e63). From my 1st couple fill ups it seems one gallon of 93octane brings the ethanol content down roughly 10 points. So I start the car with 3/4 tank (or a bit more) of e85 and my gauge reads e79%. Put in 1.5 gallons and I'm about at e63, done. Good to go.