Plane.RotationY = Plane.RotationX / Plane.Position;

You can get the length of a vector with the length( v ) function, which returns a floating point:

// The RotationY is calculated by RotationX and the distance of PlaneA and PlaneB:

// Note that we will divide by zero if PlaneA.Position == PlaneB.Position !!!

Plane.RotationY = Plane.RotationX / length( PlaneA.Position - PlaneB.Position );