Straight down is a shot I practice with and sure is a toughy. Don't think I would try it in the field butI have been known to be weak
Out to 25 yards though I pretty much always hold 1/3 up the chest. I frankly find that at those kind of distances a inch high or an inch low in not gonna make or break me. I am however just about always less than 20 ft up the tree and usually right around 15ft. Seems the higher I go up the harder it is to maintain shooting lanes and shot angles become more of a concern.