I drive around 15 miles to work, about 1/2 is in traffic, and the remainder motorway.
I average around 250 miles to a tank of fuel - around 22mpg.
Worst was 220 miles, and best was around 320 miles (long motorway journey)
I personally don't think about mpg. As you say, it's an S4. It's not going to be economical.
I looked at how many miles I wanted from a tank. If you are doing 12 miles a day, and worst case, all of it sitting in traffic, then I would guestimate that you will get around 200 miles per tank, which would be 16 days of driving.