Montecarlo analysis

I have been assigned a Montecarlo analysis/ Worst case analysis research project involving a Hybrid Low noise amplifier. I know how to run montecarlo simulations using ADS but the trouble I have is with specifying the tolerance levels for the large signal model parameters of a commercially available HEMT used in the design. The tolerance levels for passive components are usually specified by the manufacturer so they don't present a problem but active device vendors don't provide statistical data.
Does one just take a ball park figure (say 20% tolerance level on all model parameters) and run the sims? Is there a well-defined industry-standard way of going about this? Has anyone gone through a similar exercise? Any help would be much appreciated.