We propose to correct the ionization factor in the quantitative rescattering (QRS) theory for a higher precision calculation of macroscopic high harmonic generation (HHG) in a gaseous medium. Compared to the first-order strong-field approximation (SFA-1) and the Ammosov-Delone-Krainov (ADK) ionization models, the Perelomov-Popov-Terent'ev (PPT) model achieves the best agreement with the macroscopic HHG spectra of Ar based on the solution of the time-dependent Schrodinger equation (TDSE). The investigation of the ionization probability dependence on laser intensity indicates that the PPT model is consistent with the TDSE, both in the tunneling and multiphoton ionization regimes, and the SFA-1 or the ADK mostly works in the tunneling regime. This explains the difference in the total macroscopic harmonic spectra, mainly due to off-axis harmonic emissions generated by small laser intensities. The examples for Ne and Xe by using three ionization models in the QRS theory also show the significance of ionization correction by the PPT model to precisely simulate the macroscopic HHG spectra. This study will help further increase the accuracy of simulating measured HHG spectra quantitatively under a variety of macroscopic conditions.