Author here: I was being overly conservative with the 20m, yes. Assuming the cutaway view in the post is to scale (it's from an architectural firm), it shows the inner screen dome is quite a lot smaller than the exterior structure – perhaps 70-80m tall; so 40-50m might be more accurate.
Oh your statement might be correct, that the average distance to closest point on the screen probably is about 20m, and guaranteed to be less than 1 radius (more like 0.5 radius perhaps.) If the cutaway is accurate, and it probably is, the interior is smaller than it feels like. So I may be underestimating how far away 20m is. I felt further than 20m away standing on the floor, but I probably wasn’t. But, what I’m really saying, is that the direction to the closest point and the average viewing direction are only similar for the best center seats, and they’re as much as orthogonal for the bad seats, the further away from the center you sit. The average view is probably greater than 1 radius, you’re generally looking across the center of the sphere. I am picking a bit, but this is also an important part of why there are no terrible seats, the viewing experience is more uniform because the view across does not correlate with your distance from the nearest point. I think this relates to why it feels immersive and works.
