The arithmetic performed when computing how many cigarettes one has left, in relation to the intervals between smoking. Generally used to determine when one will need to acquire more cigarettes, or if one can afford to give a cigarette away.
Guy 1: "Hey man, can I bum a cigarette off you?"
Guy 2: "Let me do the cigarette math... Nah I only got three left, and I can't get to the gas station for a couple hours, sorry."
Guy 1: "Uh, okay, thanks anyway."