If you're just counting holes in a 30" circle at 40 yards, they should not vary a whole lot from shot to shot with the same ammo--assuming you're drawing the circle AFTER the shot, just in case you're a bit off target.
Something I always do is cut open a shell or two and do a pellet count rather than assuming, for example, that a shell which is supposed to contain an ounce of 7 1/2's has a pellet count pretty close to 350, which is what it should have. In extreme cases, I've found that pellet count can be off from standard by as much as 10% plus or minus, which of course will impact your % when you count holes in paper, if you don't first count pellets in the load.
