I think that there's a lot of goofy advice on this thread. Ultegra dual pivot brakes are among the nicest and easiest brakes to adjust.

1. The hex screw on the caliper arm adjusts the release spring tension. Calling it a centering screw is a bit of a misnomer. If you increase the release spring tension on that arm, it will release faster than the opposite arm. If you adjust it until the spring tensions on both arms are equal, the pads will move inwardly and outwardly together. When the brakes are released they will be equi-distance from the rim.

2. When your initially setting up the brake you want the spring adjusting screw to be about halfway out. That will give you some adjustment scope in either direction.

3. With that adjustment scew halfway out, look at the caliper as a whole. loosen the bolt that holds the whole caliper to the bike. Center the caliper with your hand and retighten.

4. Cycle the brake. If one arm moves faster than the other, fiddle with the return spring adjustment screw until the two arms are equal.