Log(R) [pc] vs. log(S_Hβ) [erg cm⁻² s⁻¹] plot for calibrators with σ_ϖc/ϖ_c < 0.1 (top panel) and σ_ϖc/ϖ_c < 0.2 (bottom panel). In both plots, the blue points are the calibrators. Their logarithmic (asymmetric) error bars reflect the 1σ uncertainties of the observed parameters. In the bottom plot, the red points are the calibrators with log M_i < −2. The red line shows the two distance scales from the calibrators, and in cyan, we plot the 2σ confidence band of the scales, representing the uncertainty of the logarithmic radius R induced by the correlated errors on and .