[SOLVED] Flyback transformer design (100-150V in, 3 outputs 35W total)

[FvM]

The substantiation of your thermal resistance estimation is (as for any empirical formula) not obvious at first sight. My thermodynamics text book says in the summary of the heat transfer chapter that the usual achievable range for convection is 8 - 15 W/(m²K). So if you use 1 mW/(cm²K) as a linear coefficient, you should be in the ball park.

You can also refer to the power dissipation estimations given by calculation tools like Epcos/TDK MDT.