Nice looking bird!
I never really monitor my temp when I am cooking spatchcocked chickens. I just bank about 2/3 of a chimney of lit coals to one side of the OTG behind some fire bricks throw some chunks on and let 'er rip with the vents all the way open. I might rotate the bird once during the cook. I don't even put it under bricks any more.
Seems to pretty reliably be done after about 50-60 minutes. Next time I do it, I might put a probe in to see what my OTG is running. I am guessing it is 375-400. Next time, I'll see if I detect a different keeping the temp a little lower.
22" OTG; 22" Performer; mini-WSM; Brinkmann gasser; UDS (on the way); Maverick ET-732