Asta is the high end top variant of Hyundai i20 which has 5 sub variants according advanced features that are:
Sportz is the main variant which comes loaded with Dual Front Airbags, Passenger Airbag On/Off Switch, Front Fog Lamps, ABS with EBD, Electrically Adjustable ORVMs, Rear Washer and Wiper, Height Adjustable Driver Seat, etc.
Asta 1.4 CRDi which comes with common rail diesel engine and features are same as above.
Asta (AT), this variant of i20 comes loaded with automatic transmission gear box and same features as above.
Asta AT (O) with Sunroof, this sub variant of i20 comes extra fitted with sunroof and its price is different as above.
For further details about the car please click on the given link in highlighted body text.
Initially, Hyundai Elite i20 Magna comes with a petrol and diesel engine but a CNG kit can be fitted with the one which has petrol engine. You will have to sacrifice with power to get more fuel economy and if you convert a petrol car into a CNG car, you may loose the warranty on the car and resale value of CNG cars is lesser than petrol cars. It is always recommended to get the CNG kit fitted from the authorized service center so that you can avail the warranty services as well. Moreover, the performance will depend on how the car is driven and maintained.
Click here to find the nearest Hyundai service centre by selecting your city in the drop down list.
How much fuel goes into a car varies on a lot of factors. Now you haven't mentioned how many km did you drive post the low fuel warning light as that could also determine how much fuel goes into the car. Even though the manual says that the tank capacity is 45 litres, the amount can vary by a small margin, give or take 2 or 3 litres. Also filling fuel in the hot day or in the night also makes a difference considering fuel expands during the hot day and contracts when the temperature is cooler. All said and done, make sure you fuel up within 3 to 4 km after the low fuel indicator light pops up.