I'll assume it's safe to assume (so if I'm wrong, I'm an @ss twice) that you're talking about the Hornady HS guage...

So the Headspace gauge inserts are too large for bullet comparing - If you're using that tool, you'll need the bullet comparator set. The headspace gauge is used to measure the headspace of the case to allow you to optimize your resizing and shoulder bump, and does not touch the bullet. The bullet comparator set would let you measure your ogive-to-base length, which is analogous to your COAL, but more relevant.

COAL is easy to measure, so we measure it. But it's largely irrelevant in the grand scheme of things.

Your seating die seats the bullet by the ogive (assuming it's a proper seating plug for your bullet), and your rifling contacts the ogive shoulder through the bearing surface of the bullet. The tip of the bullet is simply free hanging in space through it's entire life, except for when it touches your calipers. The portion of the bullet from the ogive to the tip really doesn't matter, and it's really nothing more than an opportunity to introduce error into your measurements.

You can use different tools from the RCBS Precision Mic's or PTG or Hornady's Bullet Comparators to measure your ogive-to-base length. None of them are really perfect, but all of them work well enough.

So... You do Ridge Runner's dip rod test, and you find your MAX COAL with THAT BULLET.

First - subtract your desired bullet jump from that measured MAX COAL to get your TARGET COAL.

Say it was 3" with a 10thou desired jump, now we're at 2.990" TARGET COAL.

Slip that bullet into a bullet comparator and determine the tip length vs. bullet length. Subtract that tip length from your Target COAL, and you'll know your ogive-to-base length. Seat bullets and measure them with your Hornady Bullet Comparator installed until you hit that ogive-to-base length.

Write down that ogive-to-base length for that rifle, as you can use it for any bullet you use in that rifle.

Here's the weakness of the Bullet comparators - they use a reference diameter (at least TARGET a reference diameter - however accurately they actually machined it), that SHOULD be equal to your rifle's land diameter. However, if their diameter and your rifle's is not the same, you're really not measuring the actual ogive position when you seat the bullets.

Now, the reality is that you're measuring a reference diameter on the ogive that will be more accurate than if you simply measure COAL from the tip of the bullet, since ogive inconsistencies are more rare than tip inconsistencies. So this is still a good practice. But you need to use the same comparator every time - if you have two hornady kits, unless both have EXACTLY the same diameter, you won't get the same ogive-to-base length because of the machining variation.

HNI Thread: Using oal gauge comparator 30-06

I don't recall exactly what this thread covered, but I put up a couple diagrams for using the Comparator that might illustrate what I'm talking about.