Statistical file for Monte Carlo analysis

Hi, 

 I am new to Monte Carlo simulations. I am trying to do the analysis for PUF using Monte Carlo simulation. I am using IC 6.1.7 version and UMC 90nm technology. I am having some basic questions, please clear it, 

I have written a spectre file for statistical analysis for varying tox and vth of pmos and nmos. My question is 

 1. What should be the parameter value that I have to give for dtox and dvtho?  If I give zero for these parameters, I can see that these values are always at zero for all runs( I checked it in the histogram). If I should not give zero what should be the value and how should I decide?

2. What should be the std deviation value one should provide? Is there is any limitation?

3. I am also getting some notice stating 

  Statistical parameter 'dvth0_n_10_sp' depends on other parameters, its nomial value will be calculated only once and then treated as a constant.

This is my spectre code. Appreciating your help in advance. Thank you.

simulator lang=spectre

library monteLib

section param

parameters dtoxp_p_10_sp=0 toxp_p_10_std=0.2
parameters dtoxp_n_10_sp=0 toxp_n_10_std=0.2
parameters dvth0_n_10_sp=0 vth0_n_10_std=0.002
parameters dvth0_p_10_sp=0 vth0_p_10_std=0.002
//parameters du0_p_10_sp=9.26e-03

parameters mtoxp_p_10_std=0.02
parameters mtoxp_n_10_std=0.02
parameters mvth0_n_10_std=0.0002
parameters mvth0_p_10_std=0.0002

endsection param

section statistics

statistics
{
process
{

vary dvth0_n_10_sp dist=gauss std=vth0_n_10_std
vary dvth0_p_10_sp dist=gauss std=vth0_p_10_std
vary dtoxp_n_10_sp dist=gauss std=toxp_n_10_std
vary dtoxp_p_10_sp dist=gauss std=toxp_p_10_std
//vary du0_p_10_sp dist=gauss std=20 percent=yes
}
mismatch
{
vary dvth0_n_10_sp dist=gauss std=mvth0_n_10_std
vary dvth0_p_10_sp dist=gauss std=mvth0_p_10_std
vary dtoxp_n_10_sp dist=gauss std=mtoxp_n_10_std
vary dtoxp_p_10_sp dist=gauss std=mtoxp_p_10_std
//vary du0_p_10_sp dist=gauss std=20 percent=yes

}
}

endsection statistics

section models

// removed by moderator as this might be revealing confidential IP

endsection models
endlibrary monteLib