I think you can not calculate the actual mileage of your car in local driving with total precision, especially on CNG. There are a number of factors involved,
1- quality of gas - it differs from station to station, day to day.
2- the traffic conditions - its next to impossible to find identical traffic conditions on any two days.
3- your driving style - some days you're easy going, you drive relaxed, some days you press the paddle a little more. You'll be surprised to know how much revving the engine to 5k a couple of times affects the mileage.
4- You wouldn't believe how much driving ''behind'' someone at 60-70 kmph helps decrease the fuel consumption. NatGeo claimed it to be upto 30 odd percent. You can not always control that factor too.
So I say you ignore the 10-12km deviation which you notice as long as your consumption is at par with other vehicles of the same model/make/specs.