Related Rule
Germany
Practice Relating to Rule 18. Assessment of the Effects of Attacks
Germany’s Military Manual (1992) states:
Before engaging an objective, every responsible military leader shall … refrain from launching any attack which may be expected to cause incidental injury and damage to civilian life and objects which would be excessive in relation to the concrete and direct military advantage anticipated. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 457.