If it was designed for premium, but has been running on regular, how much of the new tank of gas was the premium?

I know my '95 with the 3.5l will run on regular, but it will pull the timing back as required by the knock sensor for slightly less power and likely lower mileage as well. I drive it so infrequently now I just spend for the premium anyway.

Keep us up to date on this.

John B.