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.