> Could you please anyone help me how to find/calculate/simulate that constant
> for a given MOS process?
The constant has been documented in various forms in a number of papers. Marcel Pelgrom (note spelling) and others (see references 1,2) have devised a term "matching energy" defined as Cox*AVT^2 that has been documented to show how the term varies with process node. You might examine the figures in either of these two references to better understand the magnitude of the AVT^2 term. If you are getting models for simulation from a foundry that include mismatch parameters, you can probably simulate the mismatch for devices with different W/L to emprically estimate its value for that process.
. P. R. Kinget, "Device Mismatch: An Analog Design Perspective," 2007 IEEE International Symposium on Circuits and Systems, New Orleans, LA, USA, 2007, pp. 1245-1248, doi: 10.1109/ISCAS.2007.378336.
[2.] M. J. M. Pelgrom, H. P. Tuinhout and M. Vertregt, "Transistor matching in analog CMOS applications," International Electron Devices Meeting 1998. Technical Digest (Cat. No.98CH36217), San Francisco, CA, USA, 1998, pp. 915-918, doi: 10.1109/IEDM.1998.746503.