How to choose the correct fan switch temperature

[Robert]

Hello Guy,

How do you select the right temperature(s) for the switch of the radiator fan ?
Do you take into account the difference between the head/engine and the radiator switch-on temperature of the switch (implied by the piping lenght, engine context...)? ...?

For example: if you use a 79°C thermostat and knowing the difference between head temp and swith-on temp is 5°C, what is the correct hi/lo temp for the switch ?

Beyond the result could you give a "method" of threshold determination ? Thank you.

Robert