Taking another look at this:
The Surface parameters lists 3 surfaces: the last one is the total of all edge surfaces, the surface touching the other Wall included.
But there is the Area of the Wall parameter which will give you a correct value (in the example these two Walls are both trapezoid shaped, because they join). The other vertical edge of the Wall is thickness*height.
But I have no idea of how to decide whether a Wall has a free-standing vertical edge or not

This is as far as I got now.
Laszlo
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