You want to end up with the disc running thru the centre of the caliper – if a pad rubs when it is centred then you have a sticky piston.
What I tend to do is assemble it all with no shims and the bolts left very loose – pull the lever and the caliper will line up and then from the gap between the caliper and mounting lugs you can see the amount of shims needed
If the pad drags when the caliper is centred try pushing both pistons in and holding in the one that moves easiest – pump the lever until the sluggish pad touches the disc and then release the other piston.
Unfortunatly with bicycle discs the self centring action of the pads does not always work properly as it is often easier t flex the disc rather than moving the stubborn piston hence you sometimes need to get teh pistons moving equally manually