No need to use formula to get angle - use instructions in the "Edgar Midrange Horn" article, download it at http://www.volvotreter.de/dl-section.htm The instructions are for tractrix, but you use the same method for the conical. By using the offcut 'wedges' - you can cut it to the exact angle needed without even touching a calculator or working the math. Makes assembly a real piece of cake too if you make the jig outlined in that document. The wedges are attached to the horn sides and then cut and it forms the angle needed right then and there. Works great for me.