Truncated n-Sided Pyramid Face to Face Mitre Angles Trig Problem

I followed this (poorly diagrammed) method of working out the mitre angle between faces of a pyramid and applied it to a truncated n-sided pyramid.

It seems correct because if I then create a solid and split it as if I was making it from a panel material, the result I calculate matches the angle I get when I use ANALYSE>>ANGLE and select the 2 planar faces.

BUT… if I use the face normals and calculate the angle between them I get a different answer. Is this because I don’t have the right plane input into the Angle component?

The answer I want is the mitre angle I would have to set my circular saw at to cut the edges of the panels so they fit together properly.

Why am I getting different results? I guess I’m missing some fundamental knowledge about angles in 3d? (36.6 KB)

You are measuring two different things:

and both are correct: the angles you got are and should be complementary each-other:
42.697525 + 47.3 = 90°

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Ah! Thank you!
Easy to get them the wrong way round when they are close to 45degrees!
I guess I have to pay attention to whether the face normals point inwards or outwards too!