I agree with the paper plate size for a kill zone, but I practice for much better than that.
In the movie "The Patriot" the main character played by Mel Gibson tells his boys, "Aim small, miss small."
That sounded like some great advice to me, so my goal is for the bullseye, which is often where two lines cross on the target, and if the holes ain't dang close together, I ain't happy.
On game, the "spot" is very small.
Works for me.