If you don’t know the right pressure to set when the tyres are cold, during the first lap on the track you can set a base pressure, which we can call a "standard" value that, with tyres in dry conditions, generally ranging from 0.45 to 0.60 bar, even if it depends on the tyre model, the weather conditions and how you wish to take advantage of the tyres on the track. It is important to note the set value, because this will help us understand the right pressure to be set in the next rounds.
If you know the correct pressure when the tyres are warmed up (but not cold), you can try attempts, starting from fairly high values. You can run on the track and stop when you feel that the tyres are hot and, therefore, generate the best grip. Check the pressure and, if it is higher than the recommended temperature, lower the pressure. Then allow the rubber to cool and, once cool, check the pressure: that will be the ideal value to set, when the tyres are cold, in the climatic conditions in which you find yourself.