Rob nailed it.

The only area where I see people mess up on is yardage estimation...You need to figure out the yardage from the base of your tree to the target, not from your stand to the target.

The easiest way is to range a tree at the distance you want to know, but range it at stand level. They sell rangfinders which will compensate for this, but they're not necessary if you'll just follow my above advice.