hope you have the best time w/your chickens. i just got through putting mine to bed for the night. they are just amazing creatures. the rule of thumb is to put your roost higher than your nest boxes since they go for high ground when they go to bed at night. you don't want them pooping so much in your nest boxes where your beautiful eggs will be.
that is unless you have chickens like mine that lay their eggs outside of the coop in a little dirt hollow.