See, there's Is a few issues here.How do you know what E% you're actually getting out of the pump. It varies quite a lot. I've personally had e63 from Caltex and e93 from United.
The other week after busting second gear at the track I wanted to get all the E85 out of the tank. I drained the tank and put 23 litres of 98 in, that took it down e33. Drove around till it was "low", another 30 litres, that was e12. More driving, another 20 litres, that was e6. More driving, another 20, e4 then the final 30 litres was e1.
It took the best bit of 500 kms to really dilute it.
Going back to the E%, Matty from NSW copped a E35 tank from United's E85 bowser - wont happen often, but it could and has! The only way he knew was he had the sensor and display (although not a flex tune). Without it how would you know? Well you would, when you're picking up bits of piston off the road.