Paul, There is an error here
1. 18 high x 31 base = 35.85 m length to peak
The height of the triangle has to be measured on the curve.
Yes, but like I said it was only an approximation using 36 flat triangles

The 35.85 m is the length of the panel starting on the outer edge of the bottom of the dome (0 m height)to the point at the middle of the top of the dome (18m height)