At one point you mention a "spot" at 20 yards that you need to be able to hit. How big is this spot?

When sighting in my bow I use the orange dots for targets that you can use on rifle targets. I start out with the big dot at 10 yards and sight in trying to hit the black square in the middle. That black square is maybe 1/2" by 1/2". I can't hit it every time, but when I do fairly regularly I move back to 20. At 20 I want my arrows inside the 2" circle on the big dot and hovering close the the square. When I've got this down I move out to 30. At 30 yards I use the 2 1/2" dot and when I've got my arrows punching this I go to 40. At 40 and 50 I use the big dot again which is about 4" in diameter. Once I'm sinking arrows into that at those distances I work out to 80 using a CD. If I can consistently smack a CD sized target at 70 yards I'm pretty happy. Now these are under perfect conditions on static targets.

While hunting I keep my shots inside of 35 yards unless I have some absolutely perfect conditions then I might stretch it to 40. I say might because I haven't yet.

As for margin of error, I want to hit where I am aiming, but realize there is a lot that can happen while hunting. If I am within 2" of where I was aiming I can accept that. Other than that I'm not happy, but if it leads to a successful harvest that is what it's all about. Within reason of course.