To find out if it is 100% full or not, look at the small pressure gauge near the CNG kit. It should be around 3000 psi. What I found once was that greater the pressure, greater the consumption.
Suppose I filled Rs. 350/- CNG, car would run 136 km before totally exhausting gas supply. Suppose I filled Rs. 300/- CNG, car would run 130 km before end of gas. It means the first 6 km on first refill were costing Rs. 50/-. I found out that the sweet spot was to fill b/w 2200 and 2500 psi and get best economy.
Please do a similar experiment and come up with results.
Highly doubtful about this. Euro IV cars cars like city 2010 won't be designed for hi-octane just because this is your opinion. Please back this up with some facts.