I think you SHOULD NOT and ABSOLUTELY NOT do ANYTHING with the settings you have.
If you get 80 km mileage with AC, by just filling gas of Rs 450, its absolutely perfect.
I myself own a civic 97 VTI. I get, on average 170 to 180 km mileage without AC in local city traffic. My cylinder gets filled up in approx, now-a-days in Rs. 800 to 850.
You probably have a smaller cylinder capacity and hence getting (apparently ) less mileage.
Thats my opinion, let the expert come with more precise knowledge.