Perhaps i missed this but nevertheless- Measuring coal is not really a great way except to stay inside the mag, to ensure your seating depth are consistent I suggest you use a base to ogive measurement.
As you can tell from the fact I said exactly that, I agree with you. Still, I’ve had success doing the best I could using OAL and with projectiles that are (presumably) consistently manufactured.
I switched to the Redding seating die and reduced my OAL variation to .001. I then reseated all the bullets in the seating depth test.
I will one day level up to using a CBTO tool. Today however is just not that day. Tomorrow isn’t looking too likely either. I also don’t anneal brass, turn necks, or use a bushing die to get more uniform neck tension. There are several ways I know about to improve my loading process. I imagine there are still many others that I am unaware of. For now though, I’m gonna (probably naively) trust Hornady’s consistency and say that the RCBS die was the cause of the initial large (.005) variation in COAL.