^^^That video is a bit unrealistic, that would be like dumping a bucket of water into your average fuel tank of 60-70L.
Obviously there are limitations to all sensors and systems.
All the flex sensors I have read about so far are looking for the petrol content in the fuel mixture and they do a reverse calculation.
Ie: sensor reads 15% petrol it shows e85.
sensor reads 20% petrol it reads e80
sensor reads 30% petrol it reads e70 etc...
Another informative vid.