Related Rule
Greece
Practice Relating to Rule 14. Proportionality in Attack
The Hellenic Navy’s International Law Manual (1995) provides:
In cases where a naval operation is expected to achieve a military advantage causing incidental loss of civilian life, injury to civilians, damage to civilian objects or a combination thereof, if the level of damage can be considered as excessive in relation to the advantage anticipated, then this naval operation should be called off. 
Greece, International Law Manual, Hellenic Navy General Staff, Directorate A2, Division IV, 1995, Chapter 7, Part I, § 2(c).