Fractional entropy of a relativistically decoupled species ($s_X/s_{\mathrm{total}}=g_{\ast s,X}/(g_{\ast s,X}+g_{\ast s,\mathrm{bath},D})$)

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 $(7/4)/(106.75+7/4)\simeq 1.61\%$.