Your FMC will tell you what approach speed to use for your current weight and desired flaps setting... assuming you've set it up right before the flight.

Before you get to your approach, click the INT REF button and it will show you your approach reference, and you'll note down one side of the display it will show you what your approach should be on 20, 25, and 30 flap positions.  Fly her to the ground at about that speed, and you should be golden.