I carefully marked the middle of the 2x4 beam using both a tape measure and checking with a laser rangefinder. I also checked the distance from the center of the top beam down to the inside of the gores at the base ring. The two measurements were within 5mm of each other after several trials. The height of the dome was then confirmed at 1.5m [5'] inside.
Next I have to add short plywood arcs to complete the ribs so that they join the slit frame as a continuation of the dome's external hemisphere.
All the ribs need sanding to even out the lumpiness from roughly jigsawing the flats for the covering panels. I keep wondering if glue is necessary if I can climb on the structure without so much as a creak [from the woodwork.]
Suitably clad in an industrial dust mask, hearing defenders, goggles and a hat I sanded the ribs smooth on their edges. I used an angle grinder and 80 grit disk. By the time I had finished, the dust on my forearms was 3/4" of an inch thick! The racket was drowned out by a neighbour cutting up pallets for firewood and another resurfacing his drive with a JCB.