Sorry don't have an answer but I suggest you log this query under carpentry/joinery as a joiner will best give you the answer you need rather than a plasterer. This is the sort of problem many joiners will have come across only in wood so the same principles will apply.
Hi boab, There is a project under "angles" in the DIY PROJECTS on this site, it will give you a better idea and description with diagrams than I can!
Best of Luck
KB
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!