Since the last tank was a little higher ethanol content (4 gallons of E85 rather than the 3.5 gallons that I had been doing) and I didn't get any CELs, I chose to up the ethanol content a bit.
Gallons E85: 4.51 @ $1.75/ gallon
Gallons 87 Octane: 8.692 @ $2.05/ gallon
ethanol content of tank: 35.68%
Octane of tank: 92.883
Savings per tank compared to premium @ $2.55/ gallon: $7.954
I predict that I will get a CEL with this tank. Looking back at my notes, I believe that when the ethanol content gets to be much above 30%, it throws a CEL.