Function crit_chance_scaled_crit_damage 

pub fn crit_chance_scaled_crit_damage(
    spell: &SpellDataFlat,
    source_effect_index: u8,
    ranks: f64,
) -> Vec<CritChanceScaledCritDamage>

Reads the crit-damage terms spell states as a share of the caster’s critical strike chance.

source_effect_index is the 1-based effect the spell’s own description names for the value. Every live spell of this shape parks the percentage on a dummy effect and ships the real crit-damage effect at base value zero, because the client computes it from the two. That zero is what makes the reading unambiguous: a crit-damage effect carrying its own non-zero value, such as the racial Might of the Mountain, is a flat bonus and is left alone.