If I am wrong - just ignore me!
(Firstly I am asuming the cable is adjusted so that th e pads are close to the rim and thatth centrebolt is adjusted to allow free movement, but no rotation)
Some of these brakes have a set of washers, usually some convex, some concave.
If you loosen the nut that holds the caliper in place, this will allow you to align the pads (within reason).
Get a small piece of card and [place between the rim and back of the brake to "toe-in. THen pull the brakes on tight.
Ensure the brakes align horizontally and vertically with the rim, and are not touching the tyres..
Now tighten the nuts and release. You should find the brakes aligned.
This will optimise the performance.