If your bow is reasonably fast and you are only 12' up, it is not going to matter that much at those ranges. Specially 20 yds, you will not notice any diff at all. At 35 yards, I would doubt you could tell much either. It is when you get higher and shoot longer distances that you will notice the effects.

But your shot placement on the animal will be affected! At 20 yards you will need to aim higher on the animal to get your arrow to pass thru the largest part of the vitals. Visualize where you want the arrow to come out on the oppisite side and pick your aiming spot accordingly. The closer the animal is the higher up on its body you will need to aim.