Disc brake just a little too tight
There is no adjustment for piston position on hydraulic brakes. 

Hydraulic brakes have a built in mechanism to retract the pistons slightly when the brake lever is released. The pistons are mounted in flexible rubber seals that deform as the pistons are pushed forward. The seals then pull the pistons back as fluid pressure is released. The pistons also slip through the seals to automatically advance as the pads wear.

What you need to do is adjust the calipers with the pads advanced up to the rotor. If you push the the pistons back and adjust the caliper position you wont be able to align the pads accurately enough.

A good trick is to loosen the caliper bolts, then hand tighten them so the caliper will still move. Pull the brake lever so the pads grip the rotor, keep it pulled while you tighten the bolts enough so the caliper wont move, release the lever and tighten bolts to proper torque.

Park Tool video on how hydro brakes work: