[Halon]

Not sure how well that'd work. I see what you're thinking, because the 85% mix would probably perform better, so you'd tune it on the weaker 70% mix and just use that even when it goes up to an 85% mix. But remember that when going from 70% up to 85%, you will need to actually add more fuel because of the higher ethanol content.

My plan is to get a dyno tune done early spring on the weaker mix. By summertime I should have a wideband installed in the car. I will have to just monitor the wideband readings, and make minor changes from the original tune over the course of the season when I start noticing changes.