Is it resonable to take gasket material young's modulus as the effective gasket modulus of ring type joint in flange leakage module of Caesar-II ?

Oct'91 Coade newsletter page 5 gives practical usage limitation no 3 as follow:

Leakage for flanges with ring type joints can not be predicted.

Does the above mean that flange leakage module of Caesar-II is not applicable for ring type flange joints ?

regards,

sam