The auto temp setting is like the thermostat in your house; set the temp to whatever you want the temp to be. Unlike the blue and red colored knob on manual controls, the auto temp setting does not control the temp of the air coming out of the vents.
Beyond that basic starting point, you have to throw in a couple more variables. First, the system measures the amount of sunlight, and it will compensate for sunlight with cooler air. So it's trying to anticipate how much the sun will heat up the interior, and then it possibly blows cooler air to maintain your set temp. The lower your set temp, the more likely it will be that it will blow cool air in direct sunlight, regardless of the outside temp.
Second, the system uses the A/C compressor as a dehumidifier at an outside temp all the way down to about 35 degrees. As long as it's not cold enough outside to cause things to freeze, the A/C might be on. The light on the A/C button is almost always on, but that's not indicitive of whether or not the A/C is really running.
Finally, it will adjust the recirculating mode to provide the most efficient heating/cooling possible. For example, in the summer, it will always turn on recirc mode right away when you start the truck. That's because it's more effective to cool air you've already tried to cool, rather than cooling fresh hot air. That's necessary at least until the A/C system gets up to speed, and then it will turn the recirc off. The Owners Manual has always recommended this practice with manual A/C controls; the auto computer is just doing it as a matter of fact.
In the winter months, I've got the temp set at 74, and I never have to touch it.
