The old army way was two breaths on the 3 rd. breath let half out and then take up the 2 stage military trigger and fire. The one problem with this technique is holding air in good clean healthy lungs, The oxygen level in the blood starts raising. Then add adrenalin into the mix of excitement makes the body not want to be still, It wants to go into overdrive. In police officer training where you may have to shoot 10 to 20 rounds quickly in qualification sections, some instructors teach, breath out a normal breath, hold breath, quickly shoot a few rounds, Breath, Let out normal, Hold breath and fire again. It is not bad controlling breathing for firing the shot at elevation a person is used to. Then go to 10,000 feet or above hunting it is a whole different game for many flat landers. When hunting at high altitude, You as the shooter must control the hunt and speed of travel. I have heard many stories of guides setting a walking pace that when game is spotted the hunter is huffing and puffing till they can not do justice to the shot. I learned High Country hunting alone. Set your own walking pace keeping yourself in shape to shoot when the time arrives for a shot.