OK, guys, you may be overthinking this. Do you want to protect your transmission or determine the eficiency of your of your radiator and or your cooler?

Do you want to know the temperature of the fluid in the trans, or impute it from the temp of the fluid coming back from the cooler? Suppose you have a really, really good cooler, and all you see is 160* fluid in the return line. OK, looks good. However, that cooler is giving you a 40-50* drop, and your trans fluid is really over 200*. You're fat and happy until your trans burns up.

Why would you want to know whether hot fluid was going into the trans? If hot fluid is going in, it's surely going to be hotter coming out, because there is minimal cooling inside the trans. You can tell whether your radiator is working from the water temperature.

An automatic transmission - especially one with an overdrive - can flash excessive heat in a very short time. And, the temp IN the trans is what makes or breaks it. I want the best info at the potential point of damage.

Your mileage . . . or temperature . . . may vary.