Searched in vain for a guide on how to fit a crown molding against a slanted ceiling with a 50-degree pitch. Finally, I experimented until I found a method that worked, maybe it can help some poor soul who is googling?
There are probably other methods out there, but I didn't find them
Amateurishly and without the right tools, carved the backside with a utility knife, etc.
Caulked and painted for the best finish.
Good luck!