Calculate the cold pressure to set in your tires to reach your target pressure once hot, based on the ambient temperature and the type of tire used.
The air inside the tire expands as it heats up: the bigger the gap between the cold (pit) temperature and the operating temperature, the more pressure builds up during the stint. This calculator starts from your target hot pressure to work out the cold pressure to set before you go out.
Weight distribution and braking load the front and rear axles differently: they don't heat up to the same temperature, so they don't share the same hot pressure target. Set each axle separately.
Calculation based on the ideal gas law (pressure proportional to absolute temperature); default values are indicative, check your tire/simulator setup guide.