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Do monte-carlo analysis. Foundry like TSMC includes monte-carlo analysis in their model files. If your foundry does not provide then you will have to write it yourself. An easy way is to add a voltage source for each MOS, which will emulate the Vth mismatch. Then you should write a script where you will include the value of standard deviation. The standard deviation can be calculated using mismatch characterization provided by the foundry.