I have to agree with Brad (despite not having an auto box so having no real right to post in this thread ) - the purpose of your cooler is to keep the oil within optimal operating temp. Your thermo-switch gets the fan to kick in at a certain temp, but that doesn't mean you're suddenly going to drop the temp instantly - you'll almost certainly continue to do whatever it is that's causing the fluid to heat up (ie driving your car) therefore the fan is not so much going to drop the temp as it will slow its rise, and hopefully arrest it at a safe point well before things start breaking.
So if optimal operating temp is 80-90, you want the fan coming on at 80 to give you the best chance of keeping the fluid within that optimal operating range, 'cause the thermo switch will cut out if you get below, but it can't make the fan spin any faster if the temps keep climbing...
I have a temp sensor on the return line after my trans cooler... The return temps are often (everyday normal driving) 5-20 deg and when thrashing and/or after stalling it up I've seen it as high as about 75...