Rick, for me it's a knowing, I know I can place an arrow into it's hart/lung area and I hold my self to the yardage I am proficient at.
On bears or hogs, I want to be close, it's not about hitting them, it's about putting the arrow where you want it, making a killing shot.
At whatever yardage you can consistently hit a softball size target should be your max.
You also have to take into account the adrenalin factor, your going to be juiced up, so you got to be cool