I'm going to jump into this. This is what I do... right or wrong, I have played the different seating depth with multiple powders etc. I am starting all of my loads at a specified length. I pick a specific speed I would like to achieve or come close too. At that point I set to the book OAL. Doesn't matter what bullet or what powder. I may pick one powder, or two powders, but choose the ones that most assimilate the performance I want.
I do a ladder test with 3 shot groups moving up in .5grn increments to max or above, depending on what I am trying to achieve. I shoot the rounds and that gives me a determined measurement of accuracy for my rifle, with that bullet, and that powder. If you want to set to a specific length, then you need to set all the bullets to the exact same length with your different powder charges. If you want .002" from the lands, then you should set all the bullets to that COL.
After you find the most accurate load, then tuning starts to happen. You want to make sure that the velocity is close to or at your desired velocity. Then you can start playing with seating depth with that specific load that you chose as to being the most accurate. You might get lucky and not need any fine tuning, but that generally isn't the case. Hope this added some clarity. If not then I just made you more confused and for that I am sorry!
"Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it.  But small is the gate and narrow the road that leads to life, and only a few find it.
If you find your self in a fair fight, your tactics suck!- Marine 1st Sergeant Jim Ryfinger
Friends don't let friends develop canonitis!
arguing over the internet is like the special Olympics....even if you win, you are still...special!