Bullet seating depth????

With soft nosed bullets you need to measuere a hand full to get an average length of that bullet, they do varry. If you stick a bullet into the lands and it pulls the bullet out of the case bump it out with a cleaning rod and adjust the taansion on the neck of the case. If you get it too tight there will be bright marks on the bullet from the rifling. you will also need to bump another case. Works for me and the cost is a bullet.

Subtract from this measurement, the distance off the rifling, that you want the bullets seated. Stick one of the bullets you are working with, into the muzzle- backwards- and twist lightly, to engrave a ring around the ogive of the bullet. Place the engraved bullet correctly into a case and seat the bullet until the distance to the ring equals your desired seating depth. You need a metal caliper, not a plastic one.

I hope this is clear!

Square off the end of the rod first, then polish the outside surface until you think it is clean enough. Then fit the bushings/setscrew so it slides on the rod with a fair amount of friction, like a stop collar.
You'll also need a bore cleaning guide and another cleaning rod. I have made a bunch of homemades out of fired cases, tubing and epoxy. A dial or digital caliper is also required. But here's how I measure:

Close the bolt, carefully push the rod down to where it hits the bolt face. Then slide the collar down so it hits the muzzle.
Back the rod out about a quarter inch past max cartridge length.
Remove the bolt, slip a new bullet into the front of the bore guide, install the guide into the chamber. Then use another cleaning rod to gently push the bullet up into the rifling. Gently. Carefully slide the measuring rod back down the muzzle until it touches the bullet's nose.
Using both hands simultaneously on both rods, push back and forth, you will be able to feel the bullet engage and disengage the rifling. Finally, you should have the measuring rod in contact with the bullet, which in turn is right at perfect contact, touching but not sticking when you push the other way. Carefully take the caliper and measure from the bushing face to the muzzle. That will give you your number.
Write the number down, then do the "feel" again, then remeasure. If the numbers agree to a thousandth, you did it right. Mark that magic number on your box of bullets and in your reload records.

I then push the bullet back into the cleaning guide, pull the guide, then use THAT BULLET to set up the seating die, either to contact, or minus whatever the rifle seems to like. That case and bullet are usually set aside in a collection of dummy rounds I use for setup.
 
So far, I haven't blown myself up, and I get good groups.
By the way, this setup is a quick and dirty way to measure firing pin protrusion. Just set the collar against the bolt face, then pull the rod back a bit, drop the firing pin then shove the rod back in and measure that.