There was an article in one of the bowhunting mags years back that addressed this. If I remember correctly they recommended using a plastic soup bowl as your max group size, then at what ever max range where you can still keep them in the bowl is your limit, this is with hunting setup and from varing positions. I would also add practice some snap shots i.e. draw back with pin on target and as soon as you come to full draw shoot. Practice these at up to 20 yd.
Hope this helps