[Sorrowful Jones]

The angles are as follows: Starting at the top the angle is 60 degrees, the side angless are 50 degrees and the bottom is 70 degrees. If you are mitering the joints, just cut your miters 30, 25, and 35 degrees respectively. As for the angle for the edge of the lid, I would just guess about 20 degrees.

 

[Sorrowful Jones]

I dont use sketchup. Those are angles I used to build some coffin shaped pistol cases. As far as the math... just add up all the angles around the box. If you add 70, 50, and 60 you get 180. Double that you get 360 degrees.....which will get you all the way around. That combination of angles is not the ONLY combination that can be used, but whichever combination you use needs to add up to a total of 360 degrees. Not really sure if that was your question, but hope it helps