I built three of these larger arch ways in this home.
Room widths were all different, but the arches were the same. 3\4 step bull arch bead, mud on j bead, 90 bead.
I cut 4″ Canamold chair rail in half to crown up to j bead.
I built a tool many yrs ago that I use to build the rings around the lights.
I cut the profiles out of 3″x6″ pieces of tin.