Whatever is your lowest powder charge in your ladder test, make ten more cases of that very load. Clean the barrel with your cleaner of choice. Push dry patches until they are clean. Get it bore sighted, fire one at 100 yards, dial a correction, fire four more rounds. Give it a cool down, adjust 100 yard zero if needed, and fire four more for a group. That last one out of ten use as a sighter for your ladder test distance, adjust as needed, proceed with ladder test.
I do not want the barrel contaminated with any other powder or bullet fouling than the powder and bullet I am about to use for load testing. If you're a hand loader, own it, do not buy centerfire ammo, make it.