The difference is divided by two because the angle is shared equally between two contacting surfaces. This is the same angle which must be double beveled on the faces of the vertical struts. The sheathing panels will be laid on these 'flats'. The vertical ribs will both reinforce the ribs and provide a substantial bed for the flat, trapezoid, 4mm plywood sheathing panels.
I have bought some 43x43mm timber for some experimental struts to make up a single dome segment. However, the continuing bad weather has stopped most work on the build. I need to be outside to have enough room to cut the 9mm plywood arcs for the dome from 5' x 5' sheets.
It is not the matter of a few moments to tidy everything up between sudden cloudbursts! September 2017 in Denmark is heading for an all-time rainfall record. There was more rain on one particular day than the average for the whole of a normal September. The weather has constantly flip-flopped between sunny and cloudbursts. The regular gales don't help.
Thanks to Paul Robinson's Geo-Dome trapezium dome calculator I can find the weight of the struts.
More information here:
By a happy/unhappy[?] coincidence 43x43mm softwood [pine] weighs close to 1kg per meter, or roughly 2.2 lbs/ per m or 39.4".
Vertical struts = 16 x 4 = 64 x 64cm = 41m
Horizontal struts = 16 x 64 cm = 10.2 m
16 x 59 " = 9.5
16 x 45 " = 7.2
16 x 24 " = 4
Total length of struts = 71m
Weight = 71kg or about 156lbs. This is just the skeleton! Eek?
To which must be added the weight of the 4mm plywood sheathing, of course.
As I couldn't do anything very useful, between heavy showers, I measured up the strut lengths using the base ring arcs. Which just happen to be the same radius as the ribs, of course, thanks to the geometry of a hemispherical dome. My calculations for miter and bevel angles were close enough within my ability to measure to 1/4°. The 9mm ribs would end up 3" deep where the flats are cut away. Plus the depth of the vertical struts laid over each pair, of course. These measurements give me confidence that it will all go together as planned when I finally see some dry weather.