First off, forget getting overall length by measuring to the ogive.
The ogive is NOT a single point. I don't understand why so many people seem to think it is. Go look up the industry standard definition, the ogive of a bullet is the ENTIRE sloped / curved portion of the bullet from the point where it reduces down from full diameter, all the way to the tip of the bullet, which we call the meplat.
what you are doing with the comparator is measuring to a point ON the ogive, the point where it contacts the rifling leade. And that point will be different with every different design of bullet and every different rifle barrel.
If you want to measure from the base to the ogive, that's different. But you first need to take your rifle (assuming that's the weapon) and drop just the bullet you are using into the chamber. Tap it in lightly with a cleaning rod. That bullet is now right against the rifling in your gun. Put the cleaning rod into the muzzle and slowly drop it to meet the chambered bullet. Make a half-circle with a thin magic marker around the rod. That rod measurement is the ogive at the rifling.
I disagree. As described, your method simply will not work. It will not give you the point where the bullet ogive touches the rifling. It can't.
What it gives you is the point where the cleaning rod touches the bullet, nothing else. And the point where a cleaning rod stops against the bullet is well short of the point where the bullet touches the rifling.
DO you have a "cleaning rod" that can reach all the way down to where the ogive hits the rifling? I don't. An ordinary cleaning rod, without a tip loop/jag will have a threaded hole for that tip to screw into. SOME of the bullet tip MAY fit in that hole, with certain bullets in some calibers (more for smaller ones) but no rod has a hole large enough to accept the bullet far enough to reach down to where the bullet ogive touches the rifling.
With your method, as described, what you will get is a measurement from where the rod stops on the bullet, back to the muzzle. And, with some bullets and calibers, where the rod stops against the bullet could be the tip of the bullet itself.
Cartridge over all length is measured from the base of the case to the tip of the bullet. Measuring from the case base to a point on the ogive doesn't tell you squat about the overall loaded length of the round.
Telling people it does is giving bad information.