There is no approximation for 2RC that I am aware of. If I find anything I will upload a video later. But as of now it would be mostly iterative method to reverse engineer circuit parameters in 2RC
Interested in understanding how thevenin model works? Check out thevenin video, link in description: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-pGvvAZWaCL8.html