Règle correspondante
Practice Relating to Rule 18. Assessment of the Effects of Attacks
Nigeria’s Military Manual (1994) states:
Precaution shall be taken to minimise civilian casualties and damage and the precaution comprises the respect for the rule of proportionality (civilian casualties not being excessive in relation to the military advantage anticipated). 
Nigeria, International Humanitarian Law (IHL), Directorate of Legal Services, Nigerian Army, 1994, p. 45, § 16(b).