Direct comparison of the observed f_ion as a function of [S iii]/[S ii] within the PHANGS-MUSE Η ii regions with the age-dependent fit resulting from the cloudy models (colored lines, Eq. (17)). The colors correspond to EW(Hα), and for the cloudy model fits they have been scaled as described in the text in order to account for the large quantitative discrepancy between model and observation. The observed data shows a clear gradient, indicating that age trends do play an important role in driving the scatter in this relation. The empirical fit (dashed line) shows good qualitative agreement with the age-dependent cloudy model fit, supporting our choice of this empirical calibration. However, while the trends are consistent, a significant offset is seen between the EW(Hα) measurements and the predicted values, preventing a straightforward application of our model-driven calibrations to the data.