Maybe I'm confused... But if you measure the outside edges of two holes and subtract the diameter of a bullet, or measure the inside edges of two holes and add a bullet's diameter to get the CTC... wouldn't it just be easier to measure the two left edges of the holes, using the example photos shown, to get the CTC without the math? Try it... measure the two same-side edges, then move the calipers over the center of holes. Am I mistaken?
Any of the three would work. By doing the bullet tare for zero I get a direct number and I'm not covering up any of the hole and bullet marking.
The best firearm for self defense? Any loaded one will do.