Working in a factory repair center I get to test fire dozens of rifles a week. Many come in with just the bases and less frequently with rings attached. It is freaking amazing what you can hit at 100 yds sighting only through the two scope rings and letting your brain line things up. Very easy to keep things on a paper plate.

Same concept as a ghost ring sight, the brain figures it out.

Indeed, the human brain is surprisingly capable.
MY SOP for sighting in a rifle with newly installed scope is to set up at 100 yds and do initial adjustments via bore sighting, then start shooting. Most times now first shots print no more than 6 - 8 inches from the bullseye. Very easy from there to get finally sighted in for 100 yds.

Yes, the eye is a neatness nazi, which explains why constantantly centred reticles took off - and why you don't really need them. Some adjustable mounts and the odd shim could save you from a world of disappointment if only more makers had kept their nerve.

This reminds me of my AccuCover flip-up lens caps, which I use on my airguns, due to difficulty in correcting for parallax and cant at close range.
It's easy to adapt pre-existing scope covers using luminous chevron stickers.

http://nigelallen.blogspot.co....cucover-amazing.html
https://www.youtube.com/watch?v=au7t9EMpsek

On a slightly different tac but still related to the eye/brains ability to automatically centre something when looking through a circle, apparently the gunmakers of old at BSA in England use to straighten gun barrels after boring by belting them over lead blocks and sighting through the bore until they were dead straight. A straight bore showed a full circle as opposed to a bent barrel elliptical bore.