Cutting existing coving

[Trying not to bodge it]

I am installing a fitted wardrobe.

The room that the wardrobe is to be fitted into has (modern plaster) coving and there is enough space above the wardrobe doors to fit coving above them and thereby maintain a fully coved room.

So far so good, I understand I will have a hassle with the mitre bit, but what I can't get my head round is how do I cut the existing coving on the wall accurately so that I can have a nice joint. Would the use of a combination square be good enough?

[JRS]

So the cove is on and you want to add to it? The existing cove already on can be cut with a Stanley blade and snapped off.