There are ways to adjust the SG... but I know mine always under-reports just a touch.... it is usually within 0.5 gal out of a 15 gallon tank.

Sounds like you have a really good idea.
Use the E85 for one tank while you are there,
and then take some home with you for next time!
I think they sell yellow cans for E85, but I think it is "legal" to put E85 in a red gas can.... but it may not work well in lawn-mowers, etc. so you may wish to get a yellow gas can. I think blue cans are for kerosine. IIRC.

( I put E50 in a gas generator at a construction site and it ran fine. )

I think you will find ethanol costs about 1 cent more per mile to use in your non-flex-fuel cars. Is one cent per mile worth it to you to use a home-grown, made in the U.S.A. product?

( It's cheaper price, but lower FE will still result in 1 cent extra cost, likely, but that is my guess... and each experiment will help prove/disprove this. )

It costs more per kWh to get your power from a windmill also, about 30 cents more per day, just as a comparison.
-John