There are any number of threads on this topic, if not exactly in this forum, then in Remodeling, Home Decorating, or Woodworking. The simple answer is that a turn can be in one plane or another but not two planes at the same time. Both turning the corner _and_ turning up the slope of the ceiling in one cut is attempting the impossible; it must be treated as two turns. You turn the 90 degree corner, then with a short horizontal run (the piece will look like a wedge) miter uphill for the rake run. Leveling off for the flat ceiling is just one plane change, so that is a simple miter (measure the actual angle, calculate its complimentary angle and divide it by two for the miter setting).
Casey

Thanks again :)