How should the LOP, DAH, and Chokes be determined.

[Bill Anderson]

If you lay your shotgun on it's rib to measure DAH make sure the sight bead is off of the table. A better way, that I have found, is to lay straight edge (common yard stick works in a pinch) down the rib out over the butt stock and measure the distance from the straight edge to the heel of the butt.
For the chokes, you need to measure the difference between the bore and the muzzle to get an accurate choke constriction.
LOP is generally measured from the front trigger to the center extreme edge of the butt plate or pad.

Bill