Octane requirement question

I have a 30 gallon tank, and I save 10 dollars at fill up when I spend 3.04 for 87 octane and 10 percent ethanol vs 3.47 a gallon for ethanol free 91 octane fuel.
I fill up about twice a month, more during hunting season.
So that's saving at least 250 a year running 87 octane. And that's assuming I only fill up twice a month, like I said during hunting season I bet I fill up once a week for the whole month of October.