Cooler, cooler, cooler!! I brood chicks outdoors right in the coop (more precisely the run) with temps in the teens and twenties, no heat lamp (just a heating pad cave) and they absolutely thrive! Exposure to cool temps is very good for them - they feather faster, learn to meet their own comfort needs, and seem stronger much earlier than indoor lamp raised chicks, at least in my experience. You can't go too wrong if you think about how a Mama Hen raises her chicks. They are out and about in all kinds of weather, even at just a few days old, and just run under her if they get scared or for a quick warmup, but they sure don't stay under her for long! The're busy learning how to be chickens, after all! Right now my 3 week olds, even the Silkies, are out in the run with no heat source at all! Good luck! 
