these aren't mine, they are three methods shared with me by other guys, whose names i dont remember....

The method I use on bolt action rifles is to take a case that has been fired in that rifle and lightly bump it in a full lenth resizing die till the case neck will hold the bullet I'm woking with. But finger pressure can seat or pull the bullet. Leave the bullet sticking out of the case as far as possible. Chamber the bullet carfully and close the bolt. Open the bolt holding the case in with your fingers so that the extractor does not hold the bullet against the side of the chamber dragging the bullet out. Measure O.A.L. Repeat till you sure that your measurement is consisant. Subtract how much you want it off the lands and seat bullets to that O.A.L.
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.

This is how I seat bullets. Select the bullet with the flattest base(sharpest corners), that you have. Partially size just the neck of a case. Insert the bullet in the case, backwards. You need just enough tension to allow the bullet to be pushed into t
he case by the rifling. Insert the cartridge, carefully, into the chamber and close the bolt. Slowly extract the cartridge and measure its overall length. Repeat this several times, for consistency. This measurement is the distance from the bolt-face to
 the
 start of the rifling. Record this measurement.
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 eng
raved 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!

You can go to the hardware store and get a nice 1/8 to 5/16 rod, three feet, depending on what caliber you are working on. Also get a couple of nylon bushings that will fit the rod, and a setscrew.
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, to
uching 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.

Fellas;

If you get a Stoney Point guage for this, be very careful with it.  I found out the hard way that they are fragile and will not give consistant readings if there is even the slightest bend in the guage. 

The basic guage shape is a tube, one end of which has threads to allow you to screw on the special adapter cartridge case.  The problem lies just behind these threads.  At that point on the guage, one half of the tube is missing, so you have a short section of trough rather than tube.   This is a weak point & a very slight bend can easily occur.  To check for this, roll the tube with a case attached on a level surface.  It's easy to see if it's bent then, not so easy if you don't. 

If you don't know that it's bent you will wonder why you never get consistent readings off the guage.  If you know it's bent, you still won't get consistent readings, but at least you'll know why you need to stop frustrating yourself.

The guage is aluminum & quite brittle.  In other words, if you try to bend it back, the odds are way good that you are going to break it.  No big loss, the damn thing was bent & therefore useless anyway.

900F