It really doesn't matter which gauge you use, as long as repeated use with the same bullet, gives very close to the same result. You should characterize your seating depth as so much longer or shorter than that point, being careful to record the tool used, and the details of the technique. I think that instead of worrying about the difference between tools, you should concentrate on testing to determine which of these tools gives the most consistent results. In the short range benchrest game, the common way to seat a bullet long enough that it will be pushed back when the round is chambered, using the same neck tension that will be used for the final load, and measure the length of the round after it has been chambered and removed. This length is referred to a jam, or jam length, and seating depths are referred to as so many thousandths shorter than, or off jam, regardless of whether they are in the rifling or jumping. This method has the advantage of only requiring the shooter to test by changing seating depth in one direction, shorter. In my case, I keep track of all of this by recording the combined length of the stem and cap of my arbor press type seaters, that do not have a micrometer. Working with one bullet and a sized case. I seat long, measure the OAL ( I can use the bullet tip, since at this stage I will be using the same bullet for everything.) Then I chamber the round, (I use a dummy round if I am not at the range, with the rifle in normal firing position.) unchamber it, and remeasure the tip to head dimension. Subtracting the second measurement from the first tells me the amount the combined stem and cap will have to be shortened to seat to jam, and after the adjustment is made, I record the stem and cap dimension in my notes, as jam for that barrel, bullet, and neck tension, and if I have been working with more than one neck thickness, I note that. The notes will have the day's date so that I can I can go back and trace how it has changed over time, as more shots are fired through the barrel.
