Related Rule
Practice Relating to Rule 18. Assessment of the Effects of Attacks
Sweden’s IHL Manual (1991) states:
If the attack may be expected to entail such large losses in human life, injury to civilians or damage to civilian property, or a combination of these, that they may be judged excessive in relation to the anticipated concrete and direct advantage, the commander shall refrain from attacking. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section, p. 71.