I agree CC, I've had this discussion with people way too many times and nobody ever wants to attribute the smaller groups at distance to shooter error or external ballistics.
When talking about projectiles settling down or self stabilizing, the center of mass of the projectile is usually in line with the bore axis. It's the tip and tail of the projectile that is stabilizing to the bore axis. The pitch, yaw and roll of the tip and tail of a projectile is usually nominal in relation to the center of mass which is in line with the bore axis,.This is why at our distances, pitch, yaw and roll of a projectile has almost no effect on group size. The center of mass of the projectile is going to hit the target in the same spot whether the tip and tail have "settled" or not. I hope that makes sense...It does in my head, not sure if I articulated it properly.
With all of that being said, yes lance, you can test slugs at any distance. If you can only shoot to 25-30yds in your yard, test them at that distance, find the smallest group and then when you can, test them at longer distances. It's a lot harder to tune a gun for slugs than it is diabolo pellets, so you're going to have to work at it a bit.
Good luck. Stoti