I was thinking about this again and realized that, while my math seems to check out (thank you phred for the confirmation), my interpretation of one of the numbers was incorrect.
I said, "85% of 6.6% is 5.61%, meaning that if everybody smoked to some degree and smoking were the only cause of lung cancer, about 5.61% of the US population would get it."
In actuality, 5.61% is the proportion of the population, as it is now, that would have lung cancer if smoking were the only cause of the disease. My claim about "if everyone smoked to some degree" was a mistake.