Based on the altitude of the Sun and Moon.
In what way?
(grief this is like pulling teeth)
What do you mean, in what way? The atmoplane cannot be any higher than the celestial bodies at its peak, which I would estimate to be 3-4 Mm. Therefore, further south, it would have to be lower, but not too low or it would reach zero thickness before it gets to the edge. That is, assuming the DEF theory is true; I still tend to prefer the Greater Ice Wall, which would see the atmoplane at a constant thickness of probably a few hundred kilometres, with a DEF bow shock above that.