I just want to intervene here and say that you can find your areas E85 blend schedule pretty easily on the Internet. You should only have to test once during a blend season. Furthermore, if you’re on an E30 tune by a reputable tuner, there is built in leeway on the percentages (within reason). For example, (when my car ran) I always put the exact same amount of E85 and 93 in, regardless of what season/blend it was. In my area E85 varies from 50-60 (winter blend) and 70-80 (summer blend). No, fueling is not why my car isn’t running right now. Anyway, if you’re on an E-blend tune, it’s ok to test frequently and adjust but it’s really not needed.
Furthermore, while it’s not harmful to run a single gallon of E85 in your 91/93, you must be careful and avoid getting carried away. Octane is not the only difference between a 91/93 and a E30 tune. You must also consider the amount of fuel being delivered. An E30 tune must deliver more fuel than a 91/93 tune; therefore too much E85 on a 91/93 tune can run into lean conditions as you are not delivering enough fuel for the load/torque targets.