You may be right, I couldn't go straight to that recomendation in one of his books but I did find a reference to it almost immediately in the bookDid Carroll say that temps should be even across the tire? I'm going to need a quote on that one. I will say, there's a couple things to take into account concerning the 'Old Man'.
He spent time with every sort of car, from Formula Ford to Endurance prototypes. He was on the GT40 program, among others. The whole open/closed wheel thing doesn't really matter. The tire doesn't know the difference. At different times I've had people tell me I couldn't do one because of experience with the other. I've never understood why.
Most of his work was on bias ply tires. They tend to run much less camber than radials, so that would naturally make his temperature spread recommendations lower across the tire.
The older I get, the more I'm impressed with what Carroll did. He got all sorts of stuff wrong, but we only know this because he was one of very few of his era (or any) to actually put his thoughts in print. He started writing in the mid-70's and stopped in the mid-90's. He didn't have the data and computer analysis we have. Data systems were still pretty new when he retired from pro racing. It was all just pencil and paper. It's no real surprise he got some stuff wrong, but it's freakin' amazing at how much he got right!
" Chassis Engineering" by Herb Adams, who along with Guldstrand were the guys I followed because they were more production car oriented. I don't take Smith's recomendations lightly, but unfortunately they are becoming dated.
Mostly, they are behind in shock technology.