46 CFR §45.57

1. (a)Where the depth to the upper edge of the deckline is greater or less than D, the difference between the depths must be added to or deducted from the freeboard.
2. (b)When the Commandant or the approved assigning authority approves a location for the deckline that is above or below the freeboard deck, the minimum summer freeboard must be corrected by—
   1. (1)Adding the difference between the depth and D if the depth is greater than D; and
   2. (2)Subtracting the difference between the depth and D, if the depth is less than D.
3. (c)Except for the adjustment allowed in paragraph (b) of this section, no freeboard of less than 2 in. may be assigned.