Using range to calculate mileage is not good. You will have noticed that the reading goes in fives, so is +/-5 at best. The reading it shows is an instantaneous reading, reflecting the car’s current consumption. Thus, if you are doing full throttle blasts it will rapidly drop, but, if you are cruising on a light throttle, it will remain static, or even increase.
The best way to calculate mileage is to fill the tank to the brim and note the mileage reading. Run the car, as normal, until it’s time to refuel again. Brim the tank, noting the litres required and the new mileage reading. The consumption is this mileage reading divided by the gallons required. To convert litres to gallons divide by 4.546.
Incidentally, my March 2019 Performance DSG is averaging 36mpg, calculated from delivery in the above way, and currently at just over 5,000 miles. This is over mixed driving, probably 50/50 town/cruising, without much heavy traffic.