Breathing is very important and I like to watch the scope cross-hairs rise and fall as I breath while sighting, establishing a rhythm. For most folk there's a pause between exhale and the next inhale where scope cross-hairs "settle". This is the point at which your shot should break.
Everyone is different and you should strive for the "rhythm" first, then consistency of timing the "break" at the point in which your cross-hairs settle.
Dry fire practice on the living room floor, or anywhere for that matter, helps with the establishment of these factors into a muscle memory reflex and a confidence in your ability to coordinate the breathing rhythm and the break. Nothing can replace practice here.
Most world class shooters, and I ain't one, know at the "break" whether the shot is good, I do, which is why IMO hunting is such an excellent training tool, nuthing can replace the feeling of making a perfect shot on that big buck, when your heart is racing 90 to nuthin. And why no matter what your doing, proper breathing allows ya to do it better.
my 2cents,
Mike