For me and my smoker I use blankets to save fuel when its colder out. I cooked last weekend, it was 55 degrees. I didn't use the blanket and noticed that I had to have a larger fire and should have used them. I pay about $100 a face cord so every little bit helps. I don't think there are any averages when to put the blankets up.
I and my buddy across the street use a welding blanket during the warm up and pull it for the cook.
I leave my on in colder temps as I mostly low and slow . Buddy doesn't smoke so he pulls it.
Not sure what the book says, but 40° sounds about the range.