The recommendation I see allthe time is set your maximum range to the distance that you can hit a 4 inch target (circle) 100% of the time. The idea is that if you are this good on the range you will be able to hit an animal in the larger kill zone even when nerves, bugs, etc are distracting you. Personally, I think these hard rules based on marksmanship on the range aren't worth much. Wind, angle, deer behavior, nerves, stance, cover, etc etc are just as important when determining whether or not you should take the shot.

So... I would say you can defineately take a 40yd shot if both you and the deer are standing in a flatopen field, on a windless day, broadside shot, you are not shaking, and the deer has no idea you are there.

Good luck this season!