Dirk Of DC

Hello everyone. I replaced the EGR valve on my 2000 S430 because of a P0400 DTS and rough idling, especially when stopped with the car in Drive. The car's idle is still rough at stop lights although there is a noticeable increase in power. My questions are; How tight should I make the nut connecting the pipe to the valve housing and do I need to clear the code or reset the computer to correct the problem?