It is injurious to health to have a very cold airflow as compared to your outer atmosphere. The best way to keep the good air flow is to keep the temperature at the 24 degree Centigrade which is not harmful for health.
I live in Texas and we really know a lot (usually!) about Air Conditioning. The first answer makes sense if you convert the C. temperature. You don't want it too cold, because it will freeze up and stop cooling altogether. Keep in mind that the normal body temperature is somewhere around 98.6 +- 1 degree F. The air coming from the vents should be pretty cold to your hand, but not so cold that you can't keep your hand in it for awhile. A/C professionals usually use a special thermometer to measure the temp of the exit air. A cheap thermometer will work too. However, another good rule of thumb is this: If it keeps running constantly (assuming you have good insulation in the house!), and never turns off by the thermostat, then it isn't cooling! I have a new A/C in my apartment, and when it is running, the outlet temp is about 52 degrees F. That is COLD, and this apartment was built in 1974, so it has almost no insulation. But my A/C ran for about 8 minutes to cool it down and then turned off.