I just ordered my Scangauge and still waiting for it yet, so I'm just going by what's advertised on their website for this. But isn't one of the trip data parameters you can track "fuel used"? Is that accurate enough to know very precisely how much room is left in your tank?

I'm thinking that if I can use that "fuel used" parameter data to keep accurate measurements of my fuel usage on each tank, I can use that to get the proportions right. Then I can just keep a couple of 5 gallon fuel cans of E85 at home so that when I get to empty I know exactly how much to put in at the station and then come home and top it off with the E85. It's a bit of a trip to get to the nearest station with E85 (about 7 miles from home in the wrong direction from my trip to work), so it'd work better for me to stock up on it and mix it at homethan to go to that station each time to fill up.