To determine proper seat depth for any given projectile, keep in mind that the measurement is only valid when the contact of the bullet’s ogive and the lands/grooves is determined. Your manual says OAL is 3.020?… maybe for that bullet that they used, but only for that bullet profile, not all others. Projectile profiles vary from mfg to mfg.and application to application. So how do you do it?
There are any number of ways, but I’ve always used the same methodology. Take a sized, empty and unprimed, uncharged case, start a bullet into the case mouth leaving it protruding further than is apparently correct. Place it in the rifle’s chamber by hand, ease the bolt into full battery and “smartly” eject it. Measure that OAL and seat it 2 to 4 thousandths deeper. This is a good start. Later, when you’ve become more deeply involved in data gathering, you may want to play with seat depths to find the sweet spot for your cartridge. I have specifics I use regularly.
Yes, there are other ways. If you like your way better …….use it.
Once you determine your chambered OAL for that bullet, screw your seating die down until the mouth of an empty case stops the descent and back it out a full turn. Lock the die in place and back out the seater. Note that a good seating die will index on the ogive, not on the tip of the bullet.
That method is only going to work for one bullet profile, and it its a hollow point its not going to be accurate anway since few meplats are the same even in the same box. Your bullet seater should be indexed behind the tip ahead of the ogive, not on the tip of the bullet. A Sierra 175 MK is not going to be the same as a Berger 175 VLD at all. Both should index on an area ahead of the ogive, but not all ogives are the same distance from either the case mouth or the bullet tip. If you use just the tip you’re going to have two completely different freebores for the same caliber, weight and charge with likely two different results.
Now measure it and decide how far off the lands you want to begin. Note that figure and begin working out your load, adjusting seat depth as you go. But remember that was only for that bullet profile. Change profiles and you’re back to square one.