Back in the old days there weren't any of the wonderfull tools that we have the option of buying today. Yet proper OAL was important even then.
So what was done was to take an empty and unprimed case, and seat a bullet long, and then insert it into the gun and close the bolt. That would seat the bullet by jamming it against the lands.
Next step is place it in the die and turn the seating stem down to touch the seated bullet, then back off and turn it in a turn or so deeper.
Remove the round from the press and use a match to smoke the bullet only. Then reinsert the round back into the gun and look for rifleing marks on the bullet. Continue to turn the stem down and reinsert
the resmoked bullet until the marks barely go away.
Keep the cartridge as a gauge for that bullet for future die settings. Nothing to buy as nothing is needed. Use calipers for deeper seating depth from the touching depth just established.