Well, I f-ed up. I meant to put in 3.5 gallons of E85, and instead I put in $3.50 of E85! D'Oh! At least I'm not getting any CEL's, although I think it sounds worse under acceleration (i.e. pre-detonation)
Gallons E85: 2.204 @ $1.589/ gallon
Gallons 87 Octane: 11.536 @ 1.689/ gallon
Ethanol %: 20.06%
Savings per tank over premium @ $2.089/ gallon: $5.72
The car is running well, other than the occasional CEL, no issues.
I'm going on a trip this weekend and will run some premium (93 octane) to see how that compared to my E85 mix.