The time to shoot is when their attention is drawn to something other than you, passing car, bird, distant gun fire, whatever. Then when you shoot get on with it. Deer are experts at seeing motion, especially at closer ranges. Many will say to shoot in ultra slow motion, for myself, I find that all that does is throw me off my natural tempo and gives the deer more time to turn its attention back to me. I have shot all of my deer from the ground and most of them still hunting. When I was using a slower target form, the deer never cooperated with it. I went back to Hill form, more fluid and quicker; since I cannot control the deer, it is up to me to match the situation.