Here's what I do and I do it or four or five different calibers.
There will be fluctuations, even measuring the same bullet 5 times, you could get 5 different measurements, so it's not a hard and fast singular measurement.
I take 3 or 5 bullets, depending on time, from a single box.
I take the rod and make sure the modified case is cinched down tightly.
I then insert the case into the chamber, making sure it's butt up against the chamber shoulder. I make sure the first measurement has the 'thumb screw' in the 12:00 clock position. I tap the bullet gently, but firmly into the rifling and carefully tighten the thumb screw making sure the inner sliding rod does not back itself out, which it will. There's a finesse to this one handed procedure and you'll get the hang of it.
After I measure this first try at the 12:00 position, I'll reinsert the case into the chamber, but this time the thumb screw will be in the 9:00 position, that is to say that it will be opposite the ejection port on a RH action. I repeat the procedure and measure, careful to keep the rod/bullet snug against the rifling while tightening the thumb screw.
Finally, I'll repeat this process a third time, with the thumb screw at the 3:00 position, facing me and take that measurement.
I'll average those 3 measurements together and then start again with a second bullet at the 12:00, 9:00 and 3:00 position relative to the ejection port/action. After I'm done averaging bullets 1, 2 and 3, I'll average those three numbers together, call it a day and crack open a cold beer.
You'll be surprised by the variation you'll get. Sometimes you'll feel the bullet is against the rifling and you'll get some small ass measurement, way off the others you'll be getting.
Just try and vary the case in the chamber and cover you bases, you'll also see variations in COAL with the modified case in different positions, as all chambers are different and those cases are generically sized on most likely Hornady dies (now at least.)
Doing it this way, gives you a fairly uniform range in your measurements, due to this encompassing technique.
Chris