For the CHAOS H ii regions: f_ion modeled based on Eq. (6), assuming different input electron temperatures based on auroral line measurements (T_e([N ii]), T_e([O ii]), T_e([O iii]), and T_e([S iii])) as well as their reported 12+log(O/H) and [Ο i]/Ηα. Each of the four galaxies is shown with separate symbols, and the Spearman's rank correlation coefficient (ρ) and its significance (p) are shown in each figure. Each value of f_ion is plotted as a function of the [S iii]/[S ii] line ratio, which robustly traces changes in the ionization parameter (Kewley & Dopita 2002). We identify T_e([N ii]) as showing the strongest correlation (ρ = 0.69) and a large number of detections and therefore chose to adopt T_e([N ii]) as a reference temperature for the rest of the analysis in this paper. The dashed line shows the fit derived in Sect. 4.2, using a combination of PHANGS-MUSE and CHAOS Η ii regions. This same line is overplotted for the other panels (dashed lines) to allow comparison across the ionic temperatures.