Its not a problem and do't let anyone tell you any different...
We have a City 2010 on CNG (My dad LOVES CNG ).. And we still face the same problem... It's no big deal...
If you are indeed conscious about how much petrol your car has then top up the tank and drive...
Problem solved..
P.S: The car performs exceptionally well on CNG but just my 2 cents, drive it on petrol... This car was designed to go farther...
Cheers...