For Thanksgiving, I plan to use a recipe I clipped years ago -- John Ash's maple and soy-brined turkey that is smoked over an indirect fire on a covered grill. My question is how long to smoke the bird. The recipe says 3 1/2 hours for a 12 to 14 pound turkey; however, I'll have an 18 pound turkey. My husband thinks we should do a strict mathematical extrapolation which would mean 4 1/2 to 5 1/4 hours. Does this sound reasonable? After all of this effort, I don't want to end up with a dried up bird! Thanks in advance.