assuming the rotor is true a playing card distance would be just fine. Avid brakes look like the kind that the outer pad pushes against the inner pad that is fixed (non moving). It seems that mine however push a little too much, to the point that the rotor itself actually deforms a little until it contacts the inner pad. I dont know if this is damaging to the rotor and I have emailed Avid about the problem and havent heard back if this is okay. I will let you know what they say.