Can you share a picture of the space in question as well as share the overall design elements of the house?

Happy to share some details. The problem area is this wall that goes at a 45° angle across a vaulted ceiling, and then breaks at the top resulting in the ceiling sloping down from the wall instead of up from the top edge of the wall. I should have planned for this better. Design elements....not really a formal area at all, so heavy crown is not a necessity. I am painting all the trim and will also trim out these archs in white. I just think some molding up next to the ceiling would look nice. Any ideas?

Can you use a protractor? If so, you'll like this; the link is to info on a protractor thingie just for computing the angles to be cut. Even standard crown molding is rarely 45° for both cuts, and I think this gadget might help with your unusual angles. I admit I'd try it with elcheapo plain board first. And if you don't personally play with this sort of stuff, see if your trim carpenter would find it useful.

You do realize crown calls attention to where ceiling meets wall? Not sure you'd really like that affect in that spot.

Here is a link that might be useful: for DIY crown molding