Use the infotainment menu. Each time you take a trip, it will indicate the average trip mpg you are attaining(In Red) it will reset with each trip you take. The Current (In Blue) averages all the trips you have taken into account. You can set your trip meter in the settings to Clear the Current Values each time you push the reset button on the dash.
So based on this picture attached, if I was at the gas station to fill. I would look at the trip A meter for the miles traveled and write it down. Then divide it by the gallons you put in the car for the hand calculation. Then I would push the physical trip A clear button on the dash. This will then clear the "Current value" and start a new running average. At the same time it will move the Bar to the left as a reference to you new average. Hope this helps
The 26.6 mpg you are reading on your dash is probally the setting you have currently selected by default. If you push the "Info" button on the steering wheel the same area on the dash can do multiple functions, real time mpg, range left in miles to travel, average mpg, average speed and so on. Your mileage will increase as the car is breaking in. But the infotainment graphic seems to be the best for monitoring you fuel economy.