A calculation of the steady state temperature (max temperature) would depend on the specific heat of the oven and its contents, their masses, and the rates at which heat is added and lost. The rate of loss, of course, will vary with the environment and the temperature, and the oven can never get hotter than the heater. If you wanted to do a very simplified calculation with just water in a perfect thermos, you need 1 calorie to raise the temperature of 1 gm 1 degree C, which would be approximately 4 watt-seconds.
It's easier to just add the heat, measure the temperature, and adjust the heat input accordingly. John