38 seems to get regurgitated automatically whenever this question comes up. Especially over on the Micro Matic forums. This temperature may be fine and recommended for commercial American lagers and such accepted as gospel. But IMHO, I enjoy the flavor of my home brewed ales too much to try to kill it with cooling it to 38 degrees.
Anyway, google for carbonation chart and keg balancing. Determine the temperature you prefer, set the psi accordingly. Some people like to add more beer line to add more resistance to help with foam. I can dial mine in to work well with 5 feet of line instead of adding a big coil of hose inside the kegerator.