Specific cooking temperature affects on the traits of the food and the cooking speed, which leads to a bit of difficulty.
Also, ingredients need constantly heating.

Example: Fire pit provides 20J/t (or 20HU/t), Wok has a specific heat capacity of 40J/K, default temperature is 28°C, it will be heated up to 50°C in 44 ticks.

Recipe for food will add standard temperature and cooking time. Over the standard temperature will "overclock" the recipe, lower will be slow.

Purpose function for the progress bonus is 1 ± (Actual temperature - Standard temperature)² / Standard Temperature²(± is - whether actual is higher than standard)

Further suggestions are welcome.