The periodic particle trajectories of ions channeled between crystal lattice planes result in an oscillatory structure in the backscattering energy spectra. The yield in the first subsurface peak reaches a maximum for incidence angles ψ close to the breakthrough angle ψM. An extensive set of ψM-values for 100-350 keV protons in silicon and diamond is presented. Information about the planar potential is obtained by comparison with Monte Carlo calculations, as well as breakthrough angles calculated in the continuum approximation. The method has been applied to various low, as well as high, index planar channels and different angles with respect to the  surface normal. It has been found to give reliable results if applied to low index planar channeling when incidence along principal axial directions is avoided.