back to the OP
consider using an electric heat gun
and for the other lower temperature extreme
'Quick Freeze' spray cans
to make the bi-metallic coil
lock or unlock

compressed air expanding through a
'blow gun nozzle' will also be cooler than normal outside air

with these two ways of heating or cooling the fan clutch,
and a non-contact infrared temperature meter
I would guess you could find the lock and unlock temperatures

On a 1995 360 Magnum V8 the Dodge Field Service manual says the lock temperature is approximately 220 degrees F,
which is also where the nominal 195 degree factory Stant 'Regular Grade' thermostat with steel body and brass underarms is supposed to be fully open. 195 degrees is where that factory thermostat is supposed to just start opening.