bogden wrote:

You already know you're
trying to use a crown made for a 90 degree angle to try to use for something
other than 90

Your word picture has perhaps pointed out what I think is a
solution. Place the level piece first, relieving the top so it
fits to the ceiling, and mitering the end normally. Now make a
transition piece, mitered to the level piece and splay-mitered to
the coming rake piece.

Gonna be challenging to cut that transition piece, though.

That should work, and be geometrically correct, for inside
miters. The problem I still have is on a closet that comes off of
one level run. It's side follows the roof slope, but it's face is
level. The outside miter between the two would not seem to bow to
the above approach.

p.