The ideal spot for the tank is at the CG. That's usually too far behind the engine to work (the lines get too long). With that said, you balance the plane with an empty tank, since your tank if probably ahead of the CG. If it ain't ahead of the CG, it should be!

You said that as the tank empties, the nose drops? That tells me that your tank is too far aft. Your CG is, then, too far aft, and the plane is 'tender' or tailheavy - which in turn makes it difficult to land properly.......