Monitor Distance is a flawed way of looking at sensitivity. The proper way to compare hipfire to zoom is by using the zoom ratio between the two fields of view. If everything becomes 4.5x larger when you enter ADS, you want to reduce sensitivity by a factor of 4.5.
See: https://imgur.com/a/szjlq
This is consistent with how your brain perceives distance and size of target. E.g. a target 2x closer to you will appear 2x as large, and take 2x the mouse movements to aim at.
Zoom is the ratio of focal lengths between two images.
Skwuruhl's post in Mouse sensitivity utility by MSJ was marked as the answer