I don't know whether this is worth a thought - I did my (Victorian) house some years ago, and I used a 5" skirting mounted to the wall an inch down with the ovolo downwards and the same mounted to the ceiling an inch out with ogee outwards with a 5" plaster cove spanning the two. Only one person in 14 years noticed that it wasn't original. I'm not suggesting you use skirting, but the principle - two moulds and a spanning cove.