(a) Where D exceeds L 1/15 the freeboard shall be increased by [D−(L/15)] R inches, where R is L/131.2 at lengths less than 393.6 feet and 3 at 393.6 feet length and above.

(b) Where D is less than L/15 no reduction shall be made except in a vessel with an enclosed superstructure covering at least 0.6L amidships, with a complete trunk, or combination of detached enclosed superstructures and trunks which extend all fore and aft, where the freeboard shall be reduced at the rate prescribed in paragraph (a) of this section.

(c) Where the height of superstructure or trunk is less than the standard height, the reduction shall be in the ratio of the actual to the standard height as defined in § 42.20-40.