I disagree with John. If the smoker goes to a 140 hold temp, from 150, that's a 110° drop in a well-insulated, well-sealed box. Only allowing 5° rise is not enough. It will take at least an hour for the box to cool down to 140. A turkey is a risky smoke to hold, without opening the box. Not as much leeway as, say, a pork butt. It's more "temperature-sensitive."
I wouldn't recommend trying this with your Thanksgiving turkey, unless you've tried it before. It only takes about 4-4.5 hours to do a 14 lb bird, and I would certainly want to be there when it hits temp. But...if you really have to do it remotely, give it 10° to be safe.
The problem with this idea is you won't know what actual high temp the bird got to. For example, if you set it to drop at 155, the bird will easily continue to cook, and still may go above 165. As the smoker cools, and the cooking stops, internal temp of the bird will eventually drop, too. So, when you get home, it may show 160 as the internal temp, and you won't know if that's the high temp or the cooled temp. Make sense? Personally, I wouldn't risk a meal this important with an untried technique.
