Is that from the tip of the round to the base of the case or from the ogive to the base of the case?
COL, OAL, COAL are all used to describe the same measurement . . . case base to bullet tip, measured with calipers.
I'm guessing from the tip to the base but I've noticed that when you measure those two points you never get consistent measurements. I'm guessing because each bullet varies in length (all things being equal like case length, properly set die, etc).
I wanted to correct a notion you have here. Simply put, case length has nothing to do with COL. Two cartridges with the same COL having different length cases only vary in one attribute: how much of the case bears on the bullet. It also has nothing to do with capacity remaining in the case which is only affected by bullet length when COLs are equal. That concept was difficult for me to accept until I drew a cartridge or three on paper.
If that is true, how do you verify that each round has a consistent OAL once you've seated the bullet?
Most folks do that using COL via calipers. The rest use BTO (Base-to-Ogive) mostly with a special tool attached to your caliper.
It would make sense to me to measure base of the case to the ogive to verify consistency but then how do you determine what that equals as an OAL so you can compare it to the reloading manual?
How do you go from BTO to COL? Simply put, you get satisfied your COL is ON AVERAGE what the published load calls for, then measure those same cartridges to find the AVERAGE BTO. You now know how to move from one to the other ON AVERAGE when using YOUR TOOL. BTO measurements for the rounds coming off your press will vary less than their COL does because one of the variables (tip length) has been eliminated. But they will still vary because ogives vary . . . and there are other factors possibly in play.
IMO, people rarely if ever do that 
Most all who use BTO have previously determined how long a cartridge with THAT bullet in it must be to touch the lands.. They then adjust that number to either seat IN the lands by (eg) 0.005", or seat OFF the lands by (eg) 0.010. They don't care about COL, they don't care about the round fitting in a magazine. They are willing to single-shot in return for optimizing precision on the target.
So at the end of the day, if you are feeding from a magazine and/or not seating based on the lands, use COL trying to maintain an average close to published data, but also making each round short enough to fit in the magazine. EG, If your target is 2.800 you will often see +/- 0.005 off your press. The longest round will LIKELY fit your magazine (test that) and the rest will be fine to shoot.
If you want your seating to be vs the lands, use BTO. The published COL is irrelevant . . . your firearm has told you how long that cartridge should be.
There is a lot going on with this topic and I think I'm just generally confused. In the end, I just want to validate consistency from round to round and ensure that matches up with what the manual says....just not sure how you do that.
Good thought but consider this: The published data is based on the firearm, chamber, and barrel used as well as the specific brand and batch of components used by the publisher. IOW, it is an example. Your chamber may be tighter, your cases thicker, your powder/primer hotter, your bullet heavier, and your COL shorter. Even with all the factors going in the same high-pressure direction, such a load will likely show pressure signs but will still be safe. That's what published data allows.
And that is also why we "work up to a load" looking for accuracy while we "watch for pressure signs".
