You can never be positive, because to do so you would need to know exactly how much fuel is in your tank already, and what the E percentage in the tank is. I can always get it to probably +/- 5% so I'm happy with that.

 

yep! been filling up and filling up and im only still reading E80. bumped up little by little from 78 though so its heading in the right direction. need to find me one of those newly received shipments of E90 blends quick!