I have a plane helper that was initially put on x = 50. At first, the clipping is exactly at 50 but after moving the planehelper (by using drag controls with a Line3 so that each plane helper is moved on its path) the clipping plane is no longer the same as the helper plane location.
The further I move the plane the longer the tail is (as seen in the image). The clipping effect is at the level of the tail end rather than the helper plane. Of course, I am updating the constant of the Math Plane according to the position of the helper plane while dragging.