We investigate the radial-overlap part of the isospin-symmetry breaking correction to superallowed 0 + → 0 + β decay. The 8 sd-shell emitters with masses between A = 22 and A = 38 have been re-examined. The Fermi matrix element is evaluated with realistic spherical single-particle wave functions, obtained from spherical Woods–Saxon (WS) or Hartree– Fock (HF) potentials, fine-tuned to reproduce the experimental data on charge radii and separation energies for nuclei of interest. The elaborated adjustment procedure removes any sensitivity of the correction to a specific parametrisation of the WS potential or to various versions of the Skyrme interaction. The present results are generally in a good agreement with those already reported. At the same time, we find that the calculations with HF wave functions result in systematically lower values of the correction.