Hello,
I'd like to thank you for the specification you reported; it seems
that my formula is not properly correct!
All my doubts on that matter rose by a very particular problem I met
during my work where s-parameter simulation of a particular circuit
matches analytic calculations only using the "controversial" formula.
If I'll find few minutes, just for curiosity, I'll post the details of
problem ;-)
Regards,

Camelot