I like to make it look straight as possible as i can , but the 30cm difference made me ask this for experienced stuff
you obviously can't without something looking odd the cornice is meant to fit in the corner you can't make the ceiling look straight with it you would have to pack it down one end to make it level that means one end would be 30cm lower than the ceiling it would look ridiculous the ceiling is the only thing you can do to level it up fit the coving as normal and it won't look as bad as trying to mess about with it I have done miles of the stuff and sometimes people can't afford a new ceiling so this is the best way at least it looks neat in the corner where it should be
