I'll share the test protocol I use to determine seating depth. I learned if from an old bench rest guy, and it works pretty well.
Background info: I and others see evidence that accurate seating depths come in "nodes". For example, if you find that 0.020" off the lands works, there will be a couple more lengths that work in approximately 0.060" increments. So, 0.020", 0.080", 0.140" off the lands might all perform equally.
I start 0.010" off the lands and test in 0.010" increments. Typically, you'll find a depth that works if you try 0.010" - 0.080" off. There may well be other nodes as stated above. This is important because most factory chambers are throated VERY long, so long you commonly can't get within 1/8" of the lands. If that's the case, I'll start with the longest COAL that will fit in the magazine and step back in the same 0.010" increments.
Here's the wrinkle with the test protocol. I load 3 or 4 rounds at each seating depth, but I deliberately screw up the powder charges within those 3 or 4. I'll load one at the normal charge weight and then step back the charge in 0.5g increments. In case that's not clear, I'll load (for example) one round each at say 24.0g, 23.5g, 23.0g and 22.5g. All four are at the same COAL.
Deliberately messing with the charge weights makes it MUCH easier to see which seating depths work best. I just ran this test again a couple days ago. The worst group was 1.5". The best was 0.5". Quite an improvement!
It doesn't always work that well, but typically it's possible to discern a COAL that is detectably better.
