In all panels the solid curve shows phase-term and |$\Delta \nu$| from models along the isochrone, and the dashed line shows the same, but after applying the correction given by the power-law fit from Fig. 4. (a) Symbols show |$\Delta \nu$| and |$\epsilon$| of individual models found to align best with (lowest radial orders of) peakbagged stars, after applying cubic surface corrections based on each star’s frequencies. (b) As (a), but corrections calculated with the inverse-cubic form. (c) Symbols indicate |$\Delta \nu$| (and associated |$\epsilon$|⁠) calculated from observed radial mode frequencies (Table B2). (d) Symbols represent observed |$\Delta \nu$| (and associated |$\epsilon$|⁠) obtained by the pysyd pipeline. In all panels, the main sequence turn-off and the base of the red giant branch are indicated with arrows following Fig. 2.