I've seen several tutorials on rocket stability where one can empirically obtain the center of pressure by cutting out a 2D imprint of the rocket and balancing the 2D shape as seen in the pictures below:
What is the physical/mathematical justification for this, otherwise, extremely simple method for determining center of pressure? What are the underlying assumptions behid this method? In what cases is it applicable? When it is applicable, what order error should I expect from this method? In what cases is it not applicable?