I'd think the reference line might be the bore and not the water table. I'd suspect the standing breech and back of the barrel meet at near 90* to the bore. Possibly, the angle that's smaller than 90* has its corner at the hinge with one line perpendicular to the bore and the other touching the bottom breech end of the barrel/extractor.

If the standing breech were at an angle other than ninety degrees to the bore, and it looked like it were on face, I don't think the shell would drop in and sit flush. Containing the case head might be suspect also.