Home Forums Hunting & Shooting Ask The Gunwriters Something I've never heard before.........."seating depth first"

I'm not following the idea of determining seating depth first. How does this work using a rather generic powder charge?

I normally shot 3 shots of the same powder in 1 grain increments for most big game cartridges. I see where it groups the best, then play with seating depth and incremental powder charges to fine tune. I'm interested in how to determine seating depth first.