Hello, while I am not able to solve your problem, I can add that I also see a difference in the calculation. I only need to calculate the surface power lost (Watts), so I calculate what you call P1 and P2. We both have the same case where P1 is kinda of close to half of what P2 is. My P1 is be 73.698 and my P2 is 45.356. I am not sure where the difference comes from.