A systematic procedure is described, which provides quantitative, model independent information from specular lattice-rod scans in helium–surface scattering. Interference effects between adjacent low-index terraces are used to give a precise measure of interlayer spacings. The method is illustrated with data from experiments on the heteroepitaxial growth of Co on Cu(111). The precision and the statistical significance of the procedure are discussed. In favourable circumstances, interlayer spacings can be obtained to a precision of better than 0.02Å.