I wish I had a good answer for you - I did the same thing this year and ended up piecing triangles into the spaces - the angle divided by 2........I showed the finished tree skirt in the pictures but I really couldn't tell you a good way to do it. Sorry! Can't wait to see what you come up with.

I really must of been brain dead yesterday...1: for doing that much sewing and quilting 31/2 weeks after major shoulder surgery (paying the price today with pain) and 2: you do all mitred corners the same regardless of the angle. I got the tree skirt finished and on the tree. DH thinks it is "Cute"!