Hey BTR, Let me recommend you purchase a couple of life size archery Deer targets. Some of them have the internal anatomy drawn onto the target with light dashed lines. Just getting the "reality check" that a Whitetail is really only 3 feet tall (36"-40") helps understand a whole lot.
Also, it helps to imagine the heart about the size of a softball located between the front legs and an inch or two up from the bottom of the chest.
My favorite shot with a 243Win is "low" just behind the near shoulder so the bullet is headed directly at the off-side shoulder. It takes patience to wait for the correct shot opportunity, but it is worth it.
Another good shot is "low" directly into the chest, but the Deer tends to run a bit more when using this shot than the previous one. No gurantee though, it might just drop in it's tracks with either shot.
If you are using factory ammo, you would do well to use one of the 100gr Spire Point loads offered by all the manufactures.
Another good idea is to keep all your shots in the forward 1/3 of the Deer. When you get further back than that, you are either "Gut Shooting" or shooting at the "Wrong End".
Good hunting and clean 1-shot kills, Hot Core