Forget about the depth, PROJECTION is the only measurement to consider when cutting a mitre on any size cornice. Sit the cornice on your cutting table with the ceiling edge on the table, hold the cornice in projection (raise it to 90 degrees), providing the end of your cornice is square, measure 345mm from the end and put a mark on the front of the member thats touching the saw bench and cut it back into nothing. Just remember for an internal mitre you cut off the ceiling edge and for external mitre you cut off wall edge. Golden rule is always have your ceiling edge on the bench when cutting and always make sure its held or resting in projection whilst cutting.
This is the only way I can explain this, people who fit cornice regularly will know exactly what I mean, sorry if it seems difficult to digest.
Good luck with the job