MoD you are quite correct with this formula. However, after many domes and orbs I find that you need to in the end raise it up a little more so that it is technically incorrect but visually correct. In your example placing the pole on the top of the white highlight is where I'd put it here. This is a common error in iso. Many a dome has been topped poorly.
FYI, in my hot air balloon pixel I found that for sensible placement I had to move it even lower than normal and the place ropes to fool the eye. Domes remain an advanced iso thang...Pirate Ship Sail, a holy grail