Start loading with your longest round, and shorten the seating depth by 0.030-0.040" for each batch of five. Shoot them off a very good rest, like a front tripod and a rear bag. One OAL (seating depth) will outperform the others, probably significantly, and hopefully will also have a very small vertical dispersion component (this is the main thing you're after in this part of the test). If you are lucky, that group may also be the highest on the target relative to the POA. If you want to maximize the results, you can drill down on the seating depth in 5-10 thousandth increments. When you've found the best OAL, go back and load groups of 3-5 rounds with varying amounts of powder, but all at the same OAL. If the first test used 30.0 grains of powder, use 28.6, 28.8, 30.0, 30.2, and 30.4 for the powder test, and see if the width of the group tightens up. When it all comes together, take that show on the road.