1. With 83 you have the same 360 distance for ADS. But the FOV is different so this is in most cases not desirable. You are better off using a match based on what you can see, like MDH, MDV or viewspeed.
2. You can do like you have done, but remember that 90 in CSGO and 90 in R6 are completely different (see actual FOV).
3. The discrepancy is so small it's negligible, and there's usually nothing you can do about it. It's there to show you what differences you might experience in the sensitivity due to limitations of the game.