lna stability simulation

Here have one question about LNA stability simulation "Blf":
Is Blf=|S11*S22-S21*S12| ?. In my simulation, Blf=1.008, but S11=0.4903∠-67.145degC, S22=0.3449∠129.1degC, S21=5.31∠122.4degC, S12=0.01829∠130.4degC, |S11*S12-S21*S12|=0.26485. not equal to Blf
And how to improve Blf?


Thanks,