Related Rule
Spain
Practice Relating to Rule 14. Proportionality in Attack
Section B. Determination of the anticipated military advantage
Spain’s LOAC Manual (1996) states: “An attack is prohibited if … the damage to the civilian population and/or to civilian objects which the attack will cause is excessive in relation to the military advantage anticipated from the attack as a whole.” 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Publicación OR7-004, 2 Tomos, aprobado por el Estado Mayor del Ejército, Division de Operaciones, 18 March 1996, Vol. I, § 2.5.a; see also § 4.3.
(emphasis added)
Spain’s LOAC Manual (2007) states:
It is prohibited to launch an attack on a military objective when, based on information available in the planning phase, it could be expected to lead to casualties among the civilian population or cause damage to civilian property which would be excessive in relation to the military advantage anticipated from the attack considered as a whole. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 2.5.a.
The manual further states that the anticipated military advantage “should be substantial and relatively immediate. An advantage which is hardly perceptible or which would only appear in the long term should be disregarded.” 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 4.3.
Upon ratification of the 1977 Additional Protocol I, Spain stated that the term “military advantage” as used in the proportionality test of Articles 51 and 57 of the Protocol was understood to refer to the advantage anticipated from the attack considered as a whole and not only from isolated or particular parts of the attack. 
Spain, Interpretative declarations made upon ratification of the 1977 Additional Protocol I, 21 April 1989, § 6.