The best way is to base all calculations on a game you are really comfortable in. And it should be a game with good sensitivity settings and scaling.
Rainbow Six Siege is for instance a terrible game to base anything off of, as it scales every single scope differently. Games like Apex, BF1/BFV or Overwatch are a lot better.
If you want to base your sensitivity off of Windows, try both 0% and 100% in the link I provided earlier, one of them should feel more natural than the other.