Can't get a mental picture of what you're trying to do - level ceiling going down confuses - but no matter.
Measure the angle (135º) half it (67½º) cut joining pieces to this half-angle.
You need to do this to make the joining faces the same size - if you cut one at right angles and the other at 45º they would make 135 OK but the two faces wouldn't match one another.
If you have a more complex join where for instance the wall is bent at an angle and the ceiling slopes - down a stairwell perhaps - then you'll have to get creative - concentrate on making a good join with a short length of coving and fill the resulting gap in the straight run afterwards.
Peccavi
May 2009
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