That's one reason why I made the table in post #1.

At first tank, I can make a 30% mix based on 15 gallons.

On second tank, I will have to "estimate" based on gauges how much "empty space" I have in my tank, and make a 30% mix based on the predicted "empty space". But not knowing exactly, I will say, need to make a mix of 13.5 gallons the second time.

If I'm about to enter an area / state where E85 is hard to find, and my tank is not "empty" there may come a time when I need just 5 gallons of 30% mix to "top off".

I have 30% in my car now, and have gone 25 miles with no caution lamps or any drivability issues. Purrs like a kitten ( so far ).

Any reason to doubt the accuracy of the ScanGauge or On-board display for MPG? ( knowing each has +/- 2 MPG error on regular gas )