Your cable-actuated caliper has a fixed pad and a moving pad. Use the playing card trick, but only on the fixed side. Bike upright with the wheel in place, q/r skewer open. Slide card in between pad and rotor on the fixed pad side. Squeeze brake lever while tightening closing the skewer. Your brake should be centered on the rotor.

I've not used this method, but I have had mechanical disc brakes in the past. This is the way I would approach it. Good luck. PG