The simplest way may be to use the protractor tool with its “drag along edge” feature to measure the angle between the two planes to be mitered. The miter angle is half the angle between the two planes,measured perpendicular to the edge between the planes. The attached animation shows this idea for two rectangles that I have intentionally rotated out of alignment with the axes so you can see how the “drag along edge” feature works. In the measurements box you can see that the angle between these two planes is 150 degrees, so the miter angle will be 75 degrees.
