Comparisons between the ionization parameters measured by our plane-parallel fiducial model (U_measured) and the ionization parameters under different definitions for a spherically ionized cloud (U_X). The inner radius of the cloud was set to vary with metallicity according to Eq. (21). The ionization parameters of the spherical cloud are defined using the value at the inner radius (r_in), the value at the Strömgren radius (r_S), and the volume-averaged value, respectively. The dashed red line is a diagonal line for reference.