Question, I'm not playing with anything in the horsepower range being discussed here but when I do my load development I usually start at .010" under mag length and shoot for sd then I start playing with seating depth to fine tune as most do.
When I start pushing the bullet deeper it affects sd. Sometimes worse than others. Do you drop the powder charge to chase the sd or worry more about seating depth then fine tune the charge later ?
In my experience one directly effects the other. May not be as much of an issue in bigger cases. I'm only jacking with 300winmag and 308 mostly.
I drive myself nuts going after single digit sd. Sometimes I just have to say it's good enough for my use and go with it. If not I end up burning up all my components in development
It's not uncommon for my winmag to have an SD of 2-4. One time I touched off three in a row at 2929fps according to my magnetospeed with a 208amax, 71.0gr of imr4831 and fed.215m's.
Barrel is a 9 twist lilja finished at 26"
In my 308 I'm usually just happy as long as it's single digit sd's.