(a) The area, in square inches, of the principal display panel for purposes of type size shall be calculated as follows:

1. (1) In the case of a rectangular container, one entire side which properly can be considered to be the principal display panel, the product of the height times the width of that side.
2. (2) In the case of a cylindrical or nearly cylindrical container, 40% of the product of the height of the container times the circumference.
3. (3) In the case of a container other than cylindrical, 40% of the total surface of the container, unless the container presents an obvious principal display panel as, for example, the top of a triangular or circular package of cheese, or the top of a can of shoe polish, in which case the area shall consist of the entire such surface.