I can attempt to look through my collection of books for an official value. Yet all things considered, looking up the temperature range of the area they are native to: the rain forest in Africa, would make the most sense.
As for myself, I go based on the status of the plumage an the respiration. If the feathers are fluffed out and he's all tucked in, its likely too cold. If he's got his feathers flat and all put away like he's scared, and usually accompanied by panting, he's too hot. Also, birds use their feet to cool off or keep warm much like a dog pants or a human sweats. Body tucked down on the feet is cold obviously and standing high with the toes spread is hot.
On hot car ride I will bring a spray bottle and give them a light mist. I make sure to get under the wings and nearly soak the front of the wing where the heavy veins are. Furthermore spraying water on their feet cools them off most effectively.