Function spell_desc_crit_chance_scaled_effect_index 

pub fn spell_desc_crit_chance_scaled_effect_index(
    description: &str,
) -> Option<u8>

Reads the effect index whose value is a percentage of the caster’s own critical strike chance.

The shape is "... critical strike damage ... $sN% of your critical strike chance". The effect the percentage lands on is not named by the description; it is the spell’s own crit-damage effect, which ships with a base value of zero precisely because the client computes the real one from this statement. Only $sN qualifies: a cross-spell $<id>sN states another spell’s value, and $mN/$oN are not the base value the game scales. None means the description makes no such statement.