The link below is the target I use each year to check my gun and determine the effective maximum range!
My rule of thumb - I only count the pellet holes in the spinal cord, eye and skull/brain area only. I want at least 8 to 12 pellets in the above areas. Anything less and I move closer and try again. That wayI know I have a clean good killing shot. And that way I know the effective range of my gun.
I also place this target at the height a real turkeys head is - and I shoot from a STD sitting position.
Though the target gives you the ideal center I pattern my guns and adjust my sights so the very bottom center edge of my pattern hits the place where the skin starts and the feathers stop on the neck. That way as I am looking down the barrel sighting in on my quarry I am seeing the whole head at all times. Then I know with that neck outstrecthed my shot pattern covers the whole head.
www.mdc.mo.gov/nathis/birds/turkey/target.htm
JW