(a) People generally overestimate the health dangers of smoking rather than underestimate them. Thus, there is not a good paternalistic economic justification for banning smoking; it has to be externalities.
(b) It is extremely dubious that second-hand smoke has any but negligible health effects. There is a lot of propaganda. Read rather than cite, and read skeptically. The real reason we ban smoking is the obvious smell externality, but people want to be more dramatic and sound less trivial, so they claim they are getting cancer. For the economist, tho, the smell is a better reason-- people would pay a lot not to have to smell cigarette smoke, in aggregate--- more by far, I expect, than to avoid the second hand smoke health effects, if we could separate them.
(c) You should calculate lives lost, or total health costs, of the smoking ban. There is drunk driving. But there is also the health cost of smoking-- quantify that. And there is the health cost of drinking. Both of those are tough because of nonlinearities, but you could give it a try. Maybe for a second paper-- get this one submitted next year. The nonlinearity is that mild drinking has perhaps zero health costs, or even positive (though that is probably just bad stats), but going from 3 drinks per day to 4 has very bad health effects. The same may well be true for smoking, though I have not seen any studies of that, and it might even go the other way with cancer: going from 0 to 1 cigarette might cause more mutations than going from 3 to 4.