You'll need to give me some more information.....the box is 300mm What dimension is that? How is the picture placed in the box......I know it is diagonally, but the picture is not square, so there are TWO ways to place it in the box diagonally.....I think the clarification of the box measurement will allow me to show you how to solve this problem..... BUT the box will have to be either 90 0r 305 deep to accomadate the picture.
Although placing the picture 'diagonally' can mean the picture is standing on its corner in the box, I expect you mean it is placed across the diagonal of the box..... yes?
