Connect the enemy's armpits into a straight line and intersect the central vertical line, which is the approximate position of the heart. It can be slightly left or right. Don't hit from the side, it is likely to pierce the lungs, so as to ensure that you can see the front or back of the enemy, which is basically the heart.
4 (Sniper Elite 4 Full DLC Deluxe Cracked Edition v 1.5.0)