I just built mine, went 3 foot deep, concrete all the way to the top. Then sloped the concrete away from the post in a cone shape to keep water off. Putting in concrete olny part way, you may run the risk of moisture gathering on top of it underground and rusting. Also I primed the post with a thick coat of rust-proof primer before putting it in. Braced across the top, blew holes through and bolted on 4 horizontal 2 X 6 boards, then screwed steel roofing to that for solid sides. By the way, a 3 foot hole, 9 inch auger, 3 inch pipe, used two 80lb bags of concrete per hole on average.