Using the modified additivity rule (AR) approach, the total cross sections (TCSs) for electron scattering by large molecules C4H8O, C5H10O2, C6H5CH3 and C4H8O2 are calculated over a wide energy range from 50 to 5000eV by employing the experimental TCS results of electron scattering from atoms C, H and O. Here the atomic TCSs are derived from the experimental TCS results of simple molecules (H2, O2, CO). The quantitative TCSs are compared with those obtained by measurements and other theories wherever available. It is shown that there is excellent accordance between the theories obtained by the modified AR approach and the measurements even at energies of several tens of eV. And it is also shown that the problem that the log_log TCS curves obtained by the original AR approach are steeper than those obtained by the experiments, is introduced by the AR model itself. The intrinsic reason is that the mutual screening effects are not taken into consideration in the original AR approach at lower energies.