BikeCAD Pro exports a building specifications file which describes various interior angles. These are shown above. BikeCAD Pro also displays the actual mitre angles on screen as shown below. To understand the meaning of these actual mitre angles, consider that if one were to use a vertical mill to mitre tubes, the simplest cutting scenario would be to fix the tube horizontally on the bed of the mill and to cut into it with the hole cut saw oriented vertically. This will result in a joint angle of 90° but we would say that it was mitred with an angle of 0°. Usually, you'll need to mitre both ends of the top tube and down tube with some non zero angle. In cases where the actual mitre angle is shown to be close to zero, be sure to check the corresponding interior angle to be sure of which direction (positive or negative angle) to tilt the cutting tool.