Voila! Eight segments make a half dome. [If you have the imagination.]
The segments are sagging inwards due to a lack of support. So the final structure will be slightly taller but cover not quite such a large circle. [Nope!See below!] The squared off base of the ribs is a good indicator of whether a rib is actually level or drooping.
I struggled for an hour making adjustments and remeasuring but it wouldn't grow any taller. So I reduced my mocking mock-up to only two segments and lots of spare arcs. Finally I could sigh with relief that even I could limbo dance under the arc de triumph over adversity. Despite it seeming slightly illogical the height grows with diameter. This is because the arcs are curved not straight. Think of it more like blowing up a balloon rather than stretching the ribs further apart.
The clue was in the arrow on the right in the image above. I had been trying to fit the dome into a 1.5m radius instead of 1.6m. There is a wrap of black tape on my long measuring pole to show the correct new width. I had been using my original 3m mark instead. Later I'd made the dome 3.2m outside to allow the ribs to safely overlap the octagon. Though later again I had changed to 16 segments from the original eight after deciding on a trapezium, flat paneled dome.
Now I need to saw out the remaining  ribs to build more segments. I'll need more 9mm [3/8"] birch plywood but have all the horizontal struts ready to go. Not long now before I have a skeleton to clad. Weather permitting.
Sunday 1st October: A fine, warm autumn day with sunshine. Cut out five more ribs before running out of ply. Bought two more sheets of 9m birch ply and one of 4mm. Plus 10 more Raw-link clamps and some more, fine toothed blades for my jigsaw. I had been using a fine tooth, metal cutting blade but it has worn out, is slow to cut and is badly fraying the edges of the cut.
Changed to Bosch's fine tooth 'Laminate' jigsaw blade. Still slightly ragged on the veneer edges but much better than the old [worn] blade. Finished cutting out the rest of the ribs. [14 to leave room for the observation slit.] Followed by a quick sand with the angle grinder using a coarse flap wheel to clean up the edges.
Brushing the splinters over the edge with my hand can allow the flap wheel to clean most of them away. It's a battle between short sharp splinters or longer and softer ones which actually do much less harm.
I even tried a fine toothed, downward cutting blade but it suffered from vicious vibrations no matter how hard I pressed down on the jigsaw. The 'Laminate' blades may be the best option when they have a little more wear on them. Birch is very prone to splintering at the cut edge. Taping the cutting line might work but I'd need miles of tape.
Click on any image for an enlargement.