The commutation failure immunity index (CFII) recommended by CIGRE is an useful indicator for quantifying the immunity of LCC-HVDC inverters to the CF. It should be calculated by the electromagnetic transient (EMT) simulations under the worst fault condition that doesn't cause the CF as per its definition. Moreover, the symmetrical three-phase line-to-ground fault has been empirically claimed by CIGRE as the worst one in the previous calculation method. However in this letter, it is firstly found that the unsymmetrical double-phase fault rather than the three-phase one is actually the worst as clearly observed from large numbers of the EMT simulations. This is achieved by the comprehensive analysis of the simulated CF characteristics when taking into account the diversified fault conditions such as fault type, property, severity, initiation time. Secondly, the above interesting finding motivates a corrected CFII calculation method to be proposed considering the double-phase fault. The proposed method is further shown to be more superior than the previous method by the case studies under various ac grid strength.