Thanks!
About approximation/measurement: Yes, I also thought about this and get always confused by the different options for rotation and where I cannot manipulate a shape. Therefore I rather tried with mathematical functions and just found that I had to little information. I also thought about the sloping walls, but this would only help if one of the three angles would have 90° angle. Or do I miss something?
About the last paragraph of your post: I am sorry, but I just do not understand what you want to say and of course I want to follow your advice.