Question On Thermal Sizing Methodology
Your methodology sounds reasonable. In general I oversize power FETs as much as is reasonable so I don’t have to worry as much about problems with heat sinks.
Keep in mind that much of the heat comes from switching the FET.  Little heat is generated in the ON and OFF states; rather the heat comes while the FET is transitioning. So do what you can to have as few transitions per second. If you are doing PWM, then this usually means picking a frequency just above 20kHz so it doesn’t produce audible ringing. It also means driving the FET so it turns on/off very quickly and spends as little time as possible transitioning. There are FET drivers for this exact purpose.  Depending on your application and circuit, you may be able to avoid heat sinks all together.