I always pour the footing first, to above grade, and then secure the posts using special connectors, half of which you embed into the concrete. These connectors give you a small amount of "fudge factor" and allow you to move them around a little to line everything up and they also hold the wood above the concrete so it doesn't get wet. I don't know that any of us can determine the size and spacing of your footings, etc. without looking at your plans. You do have plans don't you? There shouldn't be much guess work involved in this process. Every place I have ever built has required plans and a building permit. You shouldn't build this thing without the proper permits and design work.