The Cook's Illustrated recipe is for only 12-14 lbs, and is 45 minutes at 425 with the breast down, and then only 1.5 hours at 325 flipped over.
If you use the standard 30-minutes-per-lb at 325, that puts an 18 lb bird at 9 hours, which can't be right.
Also, I can't tell from the Judy Bird recipe here whether the total roasting time is 30 minutes at 425 PLUS 2 3/4 hours at 325, or if 2 3/4 includes the 30 minutes at a higher temp. (Also, that recipe is only for up to 16 lbs, closer, but I'm assuming I'll still need to add some time. Question is, how much?)