Not every chicken can have the top spot, they will fuss no matter what, but judging roosting space I notice my chickens take up more than one foot of space per bird. Roughly estimate pecking reach and that's how far apart they need to be. Some will be close to each others and some will not allow it. You can try different levels till the sort it out. The lower chickens will roost according to the status in the flock. Sometimes to have to experiment. Depending on the athelic ability of your chickens is how high you want your roost. 2 feet may be maximum for a heavy no flight layer..and even then may need a step,.mine would roost in the trees if I let them. As long as they have a way up, they will climb.