Search reef central for herbie or beanimal overflow to understand better. This tou tube video should give you a better idea. Im not sure about the exact hieght of the tube height inside the overflow box but if you want to avoid the waterfall and gurgling associated with the overflow box use valves beneath the tank to regulate the flow and creat a siphon as oppossed to the durso method. I am using this method for the first time in my build but have not completed it yet, so this is all I can offer for now
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<iframe title="YouTube video player" width="480" height="390" src="
http://www.youtube.com/embed/6Pv3u0Sa1tk" frameborder="0" allowfullscreen></iframe>
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link to youtube
http://www.youtube.com/watch?v=6Pv3u0Sa1tk