Your engine temp at the manifold will run at the thermostat temp (e.g., 195) or about 5 degrees higher (195-200) if everything is working right. You want the fan to kick in if it gets to about 5-10 degrees higher than the thermostat temp, and to kick off when it gets about 5-10 degrees lower than the thermostat. With a 195 thermostat I think I would have the fan kick in at 200 and turn off again at about 185.
I've run my temp sensor mounted on the passenger side head fitting and on the intake manifold, and did not find a lot of difference in measured temp. I think the measured was always within about 5 degrees.