The 'penalty method' in the 'Pull-in of an RF MEMS switch' model

In comsol 4.4 this model provides a very valuable method to calculate the pull-in time of an RF MEMS switch through transient analysis, but I'am a little confused about the parameter 'tn=5e5 pa'( input estimate of the contact force). How can I set the appropriate value of 'tn' to make sure 'the elastic stiffness and the contact force' has been correctly tuned?
Any help is appreciated.Thanks in advance.