I don't think I'm understanding the real question. If it's a floating caliper, the caliper will self-center once everything is properly installed and the brakes applied. That assumes all the slide pins and bushings are not seized up and are properly lubricated with a high temperature grease.
Generally the brake rotor (aka disc) is fixed, so the caliper with the piston will be placed over the disc, but the "disc" will not be placed contacting the pad.
Please describe your question with a bit more detail or specifics.
-Rod