I've got the Park tools offset wrench, but I've found the old-school method more reliable for centering single-pivot calipers. It also works on all brakes of such type (the Park tools wrench doesn't always fit on the spring mount well enough):

(1) Tighten the mounting bolt nut on the back of the fork as tight as possible with the brake centered as well as you can get it as you tighten the nut;

(2) immobilize the front wheel by straddling it and holding it between your legs so that you're facing the front of the bike head tube;

(2) hold a flathead screwdriver or punch against the top of the spring on the side of the brake furthest from center. Hold it at an angle pointing down and slightly inwards, on or near the spring's coil;

(3) keeping the punch in contact with the spring, take a sharp whack at the top of the punch/screwdriver with a hammer.

This should shift the brake a bit more towards center. If it doesn't move, try the above procedure again. Repeat until the brake has swiveled a bit closer to center. Test the brake by pulling on the lever after every move to see if it remains centered. You may have to whack the brake so it actually moves past center, but then returns to center after pulling on the lever.

Two things to note when doing this: (a) take care to hit only the spring of the brake caliper. This part is steel and under tension at the coil, so the impact from the punch won't damage it, but if you hit the brake arm by accident, its soft metal will most certainly get dented or injured by the harder steel of a punch or screwdriver; (b) the pads do not need to be perfectly centered around the rim to be effective: the most important thing is that they are not rubbing against either side of the rim when the wheel is spinning freely. (Once one pad hits the rim, continued pressure from the cable will cause the other arm to swing to the rim faster, thus leading to an equalization of the brake pressure from both sides).