Chris you are using plaster coving I assume so yes mitres can be achieved to a good standard.

First work out the distance from wall to finish and from ceiling to finish then place a mark at both ends of a run, get some help and snap a chalk line from each end this will give you a working line.

Remember when cutting mitres you need to look at the cut in reverse so to speak, I made a mitre box out of ¾ ply to take the 4” cove this helps.

Once each wall is cut spread the cove edge with Gyproc cove adhesive then offer it up to the ceiling, it will take two people on long walls once in place hold it there for a few minutes or part knock in a few panel pins in wall and ceiling to hold it in place.

Smooth cove adhesive in mitre joints and sand when dry.

Hope all works out Chris