I am building lamp shades for craftsman styled lamps. I use 1/4 by 1/2 inch material lap-jointed to form the four panels, which are trapezoidal in shape. I have attached a simple shade shape and I know the angle of the slope to be about 39 degrees. I also know that to assemble the panels, I have to set my table saw to cut a miter at 24.5 degrees. When I use the protractor in Sketch up to find the slope, no problem 38.4 degrees. How do I use Sketch up to find the necessary miter angle between the panels?