Tie a string around the bead, and run it plumb down the center of the gun to the butt plate. If someone holds the string for you, you can usually measure from the centerline of the butt to the string, and get pretty close. You can run the string around the muzzle and back along the bottom of the gun to accomplish the same thing from the underside of the gun. Cast at heel is almost never the same as cast at toe.
I use string to set the rear wheel in the swing arm of my motorcycles, too. More accurate than you might believe.
Best,
Ted
