I agree with sleepless_nights. You will not even find Vth and \[ \beta \] in electrical rules. What you will find is the current at a specified (W,L; VGS, VDS). The best way is to characterise your devices by simulator and extract these parameters. It would be better if you use gm/ID instead of square law but this another story.
You can plug a transistor into a testbench. Use sufficiently large L and try to extract the parameters. If you use then Monte-Carlo simulation, you can see how much the current deviates for 3 \[ \sigma \]. Using your equation, you can infer how much was \[ \sigma \] assuming you have only one cause of deviation (dominated either by Vth or \[ \beta \] ).