We have built a large chicken coop and run, but also allow our chickens to free range on our small farm. Right now they come in and out of our door into the run, but we'd like to build a pop door for them to use instead. All the plans I've seen have the pop door elevated. Does a pop door need to be elevated, and if so, how high? And how large does it need to be? We have standard hens, no bantums. We are new at this, and trying hard to get it right for our hens. Thanks in advance!