(a) and (b) HHG yield of the 29th and 37th harmonic, respectively, obtained from the TSM approach – equation (3) – , as a function of driving peak intensity and He concentration η_%. (c) Ratio between the single-atom He and Ar HHG spectrum obtained within the 3D-TDSE. The pink and grey stars denote the ratio obtained for the 29th and 37th harmonic orders, to identify their specific contributions in panels (a) and (b).