[njk53]

The transmission fluid is at it's hottest at the stator. Unless you can place a thermocouple at the stator, the outlet side of the transmission cooling line going to the auxilary cooler or the radiator in-tank cooler is the best place. Place the probe as close to the transmission as best you can. Also, try to keep the probe away from an outside heat source such as the headers which could potentially give you a false reading.