I'd say the perfect temperature is the one allowing you to get the job done perfectly without damaging the surrounding components and the Pcb :)
I'm afraid there's no such figure in Celsius degrees on a digital hot air station screen that's good for everyone..there are tenths of possible variables..room temperature, air flow speed, hot air station precision and tolerance, size of the nozzle you prefer to use, melting point of the solder you use..only to list a few..
Just my two cents.