I'll assume you mean the Honda Civic Hybrid, as Honda has two models.
The HCH has either a 12 or 13 gallon tank, depending which emissions level you purchased. You'll have to wait for someone with an HCH to confirm, but subtract about 2 gallons from that 12 or 13 (lets say 10) and divide that number by the amount of marks. If there's 15 marks, that's 2/3 gallon per slash plus an extra 2 gallons once you've "run out".
I do not actually know when the Civic is timed in gallons to alert you to "fill up" and also do not know how many markings there are. Consider this a makeshift calculation
