Thanks for the replies, gents. Do you notice the same trend, heat gain in the center of the tun?
I'm thinking that, for example, if I'm aiming for a rest temp of 153, and the center of the tun is at 155-156 by the end of the hour, maybe I should start at 152 to account for this gain. That said, the edges of the tun drop down to about 151, so the average temp is probably right where I want it and I shouldn't change anything.