The room will be finished for the GTG with the exception of the wood ceilings and acoustic treatments. I'll use the GTG as an opportunity the find all the rattles and fix them before the ceilings go in.
still not following that at all but I am assuming you will photograph it when it goes up.
You can see on the outside wall behind the soffit there is a 2x4 that's mounted horizontally. The drywall goes right up to it. The bottom of that 2x4 is the same distance to the ceiling as the bottom of the soffit to the ceiling.
I'll attach tongue and groove pine at the bottom of that 2x4 and it will span across to the bottom of the soffit.