the shelving hung over the door uses a couple points of bearing: the 3/16" dados for the lower tier, the overlapping tabs of the upper tier, a few screws in the aft tab affixed to the upper tier mounting tab, and then a middle "septum" with wedged thru-mortises to hold the two tiers together.
I made some reference edges using some strips of mdf hot-glued to a few cast-off pieces in the shop, gluing each in situ after detaching the hand clamp here
I used a similar marking/layout strategy for the upper deck, and applied a few tabs to the side that cannot be seen between the ceiling and the wood so that it lays in place.
the next trick was to do a middle septum here, using thru-tennons with wedges to hold the two decks together. While everything was in place here, I found the center along the top, and then projected that location down to the lower tier using my japanese square. This point would act as the index mark for laying out the joinery.
a big ask, you know.
it was awkward humping the assembly up on a ladder into the position, but it tapped into place with minimal application of surly metaphors.