Is flange leakage check applicable for full face flanges? If yes, what is value of G (dia at gasket load reaction) to choose in Peq method?

I believe in C-II, for B31.3, G is taken from App-II, Table 2.5.2, which is defined for gaskets within bolt circle, ie ring type gaskets.

Can someone provide some insights / materials for topic?

Thanks in advance,
Andrew