There are several methods to determine maximum overall length for a particular bullet in a particular gun. Here is the one I like...
Pick the bullet you wish to use. Take a resized case and using a Dremel tool cut two slits in the neck down to the shoulder 180 deg. from each other. Clean up any burrs in/on the neck from the cutting of the slits. Now take the bullet of your choice and with finger pressure only, just start the bullet into the slit neck. Place the case & bullet into the chamber of your rifle and slowly but firmly close and lock the bolt. Then verrry gently, open the bolt and CAREFULLY remove the combo. With your calipers and hopefully the Hornady tool, take several caliper readings. They should be uniform. If you have some sort of bullet removing tool, collet or inertia hammer, pull the bullet. Run a couple more measurement tests exactly the same as the first one. Results should be very, very close to each other. Record that figure, which is your max. overall length for that bullet type/producer in that particular gun. Bullet on the lands. For example, if your total max length is 3.286, and you want to start at 0.020" off the lands, you need to adjust your seater die & plug so your OAL is 3.266", again with THAT particular type/make bullet in that particular gun. Record all this info and keep it filed away for future reference. (Saves a LOT of headaches in the future...). 0.010" off the lands would read 3.276". OK? Hopefully, your finished loaded rounds will fall inside the max length dimension of your box magazine. You should take into consideration the max. box mag. length as you set a seating depth.

Another method uses calipers, bullet of choice, and a long straight rod that will easily slide down the bore with a FLAT tip. The tip MUST be flat across its surface. A cleaning rod can be used, but close up the screw hole in the end as the tip of a bullet can slip inside it and give a false reading.
With the bolt closed and locked, slide the rod down the barrel until it rests firmly on the bolt face. Mark the rod EXACTLY where it exits the muzzle. I use a razorblade and slide it across the muzzle until I can score a reference line on the rod. Remove the rod, open the bolt, and insert bullet of choice into the chamber. With an eraser-tipped pencil, hold the bullet in place against the lands. Carefully re-insert the rod into the bore until it just touches the bullet tip. This is why the rod MUST have a flat end.. Using the same procedures as before, carefully mark the rod at the exit from the muzzle. Measure the distance between the marks. This is your max. OAL for that particular bullet. You can now use the procedures described above for setting a distance off the lands. Personally, I perfer the first procedure....