Bro honestly i think u are also aware with the market as well.
You have you own calculation formula I didn't understand how your calculation calculate the price of car as 680. Kidding :).
I offered you 6.25 only because of you car condition otherwise I have couple of options for Honda City 2004 wd Cng between range (600-630k).
I am willing to pay more than of its market value just becaue of the condition of car.
Rest this is your car and your decision. No offense I second Maverickk suggestion to u don't think and let the car go at 665 if true. Because wd cng it will be around 700K+ which is too too much.
Anyway, Good Luck wd your sales.