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Given the desired temperature differences, breast around 160-165 and thigh around 180+, I always ice the breasts for around 20-30 mins before putting the spatchchix (or turkey) on the BGE. Works for me, although the above mention of putting the legs to the back (slightly right of center) is a good way to take advantage of the known BGE hot spot. As long as you are cooking to temperature you will do fine-and once you go spatchcock you will never go back:)
I have done 3 spatchcocks in the last few weeks and my favorite rub was the Dizzy Pig Tsunami Spin. That was some good chicken! B-)