= Fractional entropy of a relativistically decoupled species
{title2=$s_X/s_{\mathrm{total}}=g_{*s,X}/(g_{*s,X}+g_{*s,\mathrm{bath},D})$}
If a species and the remaining bath share a temperature at decoupling, their initial entropy ratio is the ratio of their entropy-weighted degrees of freedom. Subsequent conservation of each sector's comoving <entropy> keeps that fraction constant, even when the bath heats relative to the relic. For a two-state neutrino species leaving an ideal Standard Model bath, the fraction is $(7/4)/(106.75+7/4)\simeq1.61\%$.
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