My unbalanced eyesight makes it difficult for me to get scope recticles displaying in the truly vertical/horizontal when installing/re-installing a scope. For those scopes I have mounted correctly, I want to make an index mark on the bottom rings and on the scope so when switching scopes around I just have to match the index marks and tighten the rings. I'm wondering what some might use for making the index marks.It has to be reasonably durable but not permanent, like scratches or punch marks, which would lower resale value.
You would also want marks to be sharp and
well defined. Is anyone using something, like maybe some kind of marker pen, that works ok?

How about using masking tape to mark off a narrow hash mark - then fill in with light colored nail polish? Should be durable and removable.

Thank you for the reply. It should work well, and you could have different index marks of different color when different rings are going on the same scope.
I had assumed there would be legions of shooters using some tried and true system that somehow I'd never heard about.
For people with a bunch of guns, they usually have many more guns than scopes, so scopes can get switched around alot, especially the higher performing ones. So non-permanent yet durable index marks can save a lot of time. If I were a scope and rings manufacturer, I'd have permanent index marks, on the scope belly longitudinally, to match marks on the bottom rings.

Go to an arts and crafts store and buy a silver paint pen. Yup, it's a pen that writes in silver paint. Use regular Scotch tape to mask your lines on the rings and scope, and 'write' your lines in.  The paint comes off with a quick swipe of acetone. I use it to mark my scope screws to indicate if they're backing out.