What I did to find my OAL, was after I had fired some brass to my chamber, I neck sized them only. But just enough that you can put a bullet in the neck and it not fall out and is somewhat easy to push in, but still enough tenson. Then chamber the round in the gun and then when your taking it out, be careful not to push the bullet into the case anymore and then measure it and subtract .010, ten thousands, from that number and your good to go. But as long as you get the number after pulling it then you can go from there. I did it a couple of times just to be sure it was right. Sometimes the bullet will get stuck in the lands, but big deal. Here is the video to were I learned how to do this:
YouTube - Making a Overall Length Guage
or just search youtube for making a overall length guage. Hope that helps any.