Shooting straight down is a low percentage shot. In the 45 years of bowhunting I have seen tons of deer lost with this kind of shot. The ribs have to much angle and deflects the arrow. also the rib'sarethicker the closer to the spine and it's hard to get both lungs. wait till they are 15 to 25 yards from your tree so you can get a good shotto take both lungs. Your not every high so maybe 10 yards you can get both lungs and you will have a bigger target to shoot at.