I do seating first, I just did a 300 Win with 208 ELDs. I didn't load that many lengths. I did 5 sets of 5 rounds, starting at Hornadys book length which was 3.34" went to 3.58" I think. This is all off of memory, my notes are in the basement.
Two lengths showed promise with 5 shots and they were .040" apart. I chose the one seated deepest so I could chase the lands as they get farther away. I made up dummy rounds for each length and I adjusted my die to the desired length, and then measured off the ogive, recorded it, and measured distance from the lands with a OAL gauge and recorded it.
This sounds like a lot of work, but I shot a total of 18 rounds. I didn't shot a whole string of 5, if ES and groups were poor in three shots. I have shot 40-50 rounds doing powder charge work ups, and often didn't get desired results. So it's always seating first for me.