Resize object problem

Hi all,

Basic idea is that I am able to measure circumference of the object. First I have created a plane, then put it where I would like to take a measurement intersecting with object. I used bool intersection in order to create object which I would like to measure. I am able to take a measurement by script, and everything looks fine. Based on some input I would like to resize the object and take measurement again. I parented cube to the plane that I am taking measurement on.

Currently when I scale object (Plane) in the Object mode, cube is following the plane scale but value of the circumference stays the same, even though size has changed. I am not sure why, is that.

On the other hand when I scale object (Plane) in the Edit mode, then value of the circumference is changing, but the cube is not following scale of the plane.

My goal is to resize the Plane and have the Cube following the new size of the plane, but I have to be able to take correct measurement of the circumference.

I attach my testing file.

cube_test.blend (111.6 KB)

Thanks in advance for help!