The perches or roosts should be higher than the nest boxes. So if you put your nest boxes 12" off the floor, be sure the perches are higher than that (actually higher than the roof of the nest boxes). We are putting our nest boxes on the floor because we're putting a window in above them. I think our coop will end up being more windows than solid wall. lol. So our perches don't have to be too high (probably will be about 30-36" off the floor).
They seem to like to go as high inside the coop as the roof will allow. I have a graduated roosting poll system starting about 3 feet off the floor, then 4 feet and the last on is about 5 feet high. They all compete to see if they can get to the highest roosting poll ( which is large 2-3" diameter bamboo polls). The higher the better it seems. My nest boxes are two rows, one upper row and one lower row. The first level is about 6" off the floor. The second level is about 20" off the floor. Seems to work well. The hens cannot roost on the nest boxes because I slanted the top at such a sharp angle that they slide off if they try to fly up on top of the nest boxes.