Don't be obsessive about the perfect heart shot. As others have said, it's low and forward and in the heat of the moment can be missed or a non fatal shot made.
It's good to aim small but it's not the only option you have.
Look at the animal's body. Draw an imaginary line down the middle of it and put a bullet in the middle of the front half.
You will hit the lungs, sever arteries and it will be a fatal shot. A bit low you get the heart, a bit high you get the spine.
If you have a full frontal shot, shoot for where the neck joins the chest. A bit high you get lungs, a bit low, you get heart.
Quartering shots away, just shoot for the opposite shoulder.
I've attached a photo of a kudu heart.
