Thermal switch temperature range?

[bobthompson]

I am looking to fit a thermal switch for the electric fan on my trials Chummy. It will go in the hose from the head to the top of the radiator. I wonder what is the optimum running temperature?
The options available are:
On 85C, off 77C
On 90C, off 80C
On 95C, off 85C
I am favouring the middle one, but what experience does anybody have?

Thanks,
Bob.