4 The area not including the top, bottom, flange at top and bottom of cans, and shoulders and neck of bottle and jars shall be determined in the following manner, namely:-
In the case of a rectangular package, where one entire side can properly be considered to be the principal display panel side, the product of the height multiplied by the width of that side;
In case of a cylindrical or nearly cylindrical package, 40 per cent. Of the product of the height of the package multiplied by the circumference;
In case of any other shaped package, 40 per cent. Of the total surface of the package, or an area considered to be a principal display panel of the package.