Shoot multiple rounds at each charge, but not all of a particular charge in a string. Keep the same POA for all shots. 200-300 is where you'll start to see some decent spreading, yet still be able to keep it on the broad side of a barn. The farther the better.
the hard part with this testing is spotting which hit comes from which load. The basic thought says that the higher the charge, the higher the impact on the target. That's difficult to do if you've got an SD of 40 and an ES of 70.
For what I'm trying to get done, I'm not concerned with 100 yard shots. I want to know how this ammo/gun combo does at distance.
Shoot one round from each charge weight, then repeat. Take note of where every round hit before you take the next shot.