I think, I am able to solve for it. The formula is presented on pg 114 in Kundur's book. The results are as follow S(1.0) = 0.962301 and S(1.2) = 0.817877. Please let me know if you have different answers.
See the following manuscript. It provide a helpful simulation and programming sight for Small Signal Stability studies based on Professor Kundur 's worthwhile book.
Regards,
Morteza Shabanzadeh
http://mortezash.blogfa.com/
Data Small-Signal Stability Analysis using MATLAB Programming (in Persian)