To determine the headspace, I just take an empty case, and I make a cut over the complete length of the neck of the case with my dremel.
Then I just push in a bullet a few millimeters, and I insert the case with the bullet in the chamber of my gun. Then I close the bolt of my rifle, so the bullet gets pushed into the case by the lands & grooves. Now just eject the case from your rifle. You should make your rounds a litlle shorter than this one.