Related Rule
Germany
Practice Relating to Rule 21. Target Selection
Germany’s Military Manual (1992) states:
Before engaging an objective, every responsible military leader shall, when a choice is possible between several military objectives of equal importance, engage that objective the attack on which may be expected to cause the least incidental injury or damage. 
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 Konflikten – Handbuch, August 1992, § 457.