Great Room - recessed ceiling

[Delain]

I've placed soffits around the perimeter of this room to create the first level of a tray type. And a ceiling plane at 144" which represents the upper flat or horizontal section of the ceiling.

How can I create sloped ceiling section(s) from the top of the soffits to the edge of the ceiling plane without changing the current size of the flat section at 144"?
I've drawn in two cad lines on the saved cross section view to emphasize what I'm aiming for. I've tried ceiling planes, but struggling to make them behave. Included here, are two example images that show the slanted sections I'm trying to mimic.

Thanks in advance,

Delain

great room ceiling v1.plan

[solver]

i did not look yet at your plan, but this is what I would do.

 

Take a cross section. Use CAD lines and the angle measurement tool to find the slope. Draw in a ceiling plane.

 

Review in cross section. Adjust as needed.