10/16 = 0.625m per side [or gore width.] My lower struts are 60cm in maximum length. So the ribs are adding 2.5cm or 1" each. Since they are only 9mm thick and doubled it ought to be 18mm or 3/4". 16 x 6mm [difference] = 96mm or 4". There is obviously some extra spacing spread around the bottom ring of ribs and struts. My long, alloy measuring pole shows an excess on the diameter too.
It turned out the dome was oval but I was able to pull it back to round with another ratchet strop across the widest measured diameter.
Faced with a bunch of wobbly ribs I needed a datum point at which to measure for new horizontal struts. So I just measured up 60cm from the base struts and marked the ribs. Then measured horizontally between the marks and wrote down each measurement. Dividing the total by 16 gave me my average distance. This turned out to be 55cm. Rather longer, by 2cm, than my original lower struts at 53cm.
The only way I can reuse them is to raise them considerably. Which I really don't want to do. For both reasons of economy in cladding plywood and appearance. I shall just have to buy more 45x45mm batten and make a lot of new, much longer, lower struts ti be fitted at the 50cm height. The 53cm struts can be used in the next ring upwards after trimming to avoid waste.