Like this should be good: https://www.mouse-sensitivity.com/?share=568d8bbf273c50cca684e75fdbb4c702
The ADS and Scope conversion is set to MDV 0% with a scale of 91%, this makes the calculated ADS sensitivity for R6 65 like you have it.
It depends, some do prefer a change when changing monitor size, others don't. If you want to get really technical you should also consider the distance from your eyes to the monitor as well, as the the angular size of the monitor changes.
I know... It's amazing that a game series that literally focuses on sniping just have a slider with no values. It's actually possible to count pixels (or rather every single notch on the slider) to set the sensitivity more precisely. I can add calculation for that too if it's of any interest, but it's kinda hard to set it this way without scripts/macros that move the mouse exactly one count.