Use the clinometer the measure angles along all edges of your phone. You can set a target angles. Approaching the target angle or major 45°-steps a high-precision scale will be displayed. A level feature is built right into the iOS Compass app through which you can gauge the angle of a surface. If you want your phone to do more, you can grab any number of apps from the App Store, like handy Level and Bubble Level for iPhone. The formula involves dividing 360 by the number of sides to calculate the corner angle. Then divide it by two to get the miter angle. If you're making a five-sided project with all sides equal, you divide 360 by five to get 72 degrees. So each joint or corner forms a 72-degree angle.