I would create a MathPoint with coordinates (0,0,1)
Then use the MathTransform from ISketch::ModelToSketchTransform to multiply that point. The resulting point is the normal
Here is the good example which is doing exactly what you want: http://help.solidworks.com/2013/English/api/sldworksapi/get_the_normal_and_origin_of_a_reference_plane_using_its_transform_example_vb.htm
Use the same techniques to get the normal from transformation for ref plane and sketch.
Artem Taturevych, Application Engineer at Intercad (Australia)
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