You might find it helpful to look at the code for Angular Dimension 2 or even the the original Angular Dimension plugin by @slbaumgartner (Steve Baumgartner). He wrote the code to measure the angle between two edges given three picked vertices, the middle pick being the common vertex. I think you could supply the equivalent from two planes with a common edge.
They are free, from SketchUcation Plugin Store.
I think that SU
angle_between always returns an angle less than 180 so you have to figure out when to add 180.
And I see that 200.3 = 180 + 20.3.
And also that if you draw the normals in your diagram with a common vertex on the edge, you would expect to get 20.3°, not 159.7°, wouldn't you?
If you want to get the obtuse angle, try reversing the direction of one normal.