Centerline of rectangles (Wall centerlines)

Good afternoon ladies and gents, I've been trying to get the centerline of my shear walls which appear as closed curves in Rhino.

I've been tinkering around with GH but I can't get the centerline I want. Is there a way in GH to form a line between the 2 shorter sides in a rectangle?

Picture shown below is the GH file that I made but requires adjustments with the number slider and also it forms the wrong centerlines for the walls depending on the orientation of the rectangles.

Ming