I'm building a good sized coop with a grand run. I want my girls to have plenty of room to roam and not end up in the neighbors yard. The run itself will be 30'x20'. It will also have a large tree right in the center of it. Due to the run size and the tree, I will not be enclosing the top of the run. My question is how tall should I make the fence so the girls don't get out?