If you notice in the photograph, the molding isn't set in relation to the fireplace per say. The line is an extension of the crown molding along the short walls (along the left and right in this photograph). You could try running painters tape across your end wall, from the corners where the slope begins, and see if the effect is visually appealing to you.

Can you refer me to who the talent was behind the trim for Large Oval Window? Thank you

Hi! Our office executed all the drawings and designs for the millwork in the project, and a local millwork shop fabricated the trim to our specifications.