The ideal temperature for almost all automatic transmissions is 140 degrees, that's where most fluids function best, at this temp ur tranny will work great and last an awful long time. It is hard to keep the temp that low in the real world though. The best way to hook up an aux cooler is AFTER the radiator trans cooler in series, ur thermostat(190) keeps the engine temp optimal, but leaves the transmission fluid temp too high. Run long lines to a remote aux cooler location, and then back into your tranny, (the long lines actually aid in cooling the fluid) its almost impossible to run the fluid to cool, maybe at the nrth/sth poles. 140 perfect world, 170(achievable)190 max, 230 shut her down.