Y2K - welcome to the wondeful world of bow hunting! There's no exact science to it... a lot of it is experience and lot is luck. If a deer is trotting towards you, you're going to have to have seen or heard him in time to get to a full draw THEN grunt or whistle. If you grunt or whistle before you draw they'll be looking right up at you and you won't stand a chance. A lot of this depends on how long you can hold your bow at full draw and still shoot accurately. If you know you can hold for quite a while you can draw back when you're confident they are heading you're way and just wait. If you're going to wait until they get close you have to make sure their head is either behind a tree or turned away from you. A lot of this can be helped by stand position also. Try to position your stand so that the trail naturally leads the deer behind a tree or brush that would hide your movements and give you a chance to draw. Then you can shoot when they hit the opening. I hope this helps...just keep learning from every situation you have with a deer and you'll be fine.