Why the devs made it go from -10 to 10 I don't know, but 0 is 100% monitor match.
Preferably use hipfire if that's what you base your ADS on. That way you won't pass on any discrepancies due to decimal places etc.
Added now, but note that FOV calculations only works from multiplier. Selecting a different FOV Type does not work until I update some code.
I don't make such tools myself, but if there is one available I'd be more than happy to try it out.
For most games the calculation is 100% accurate. Some games do have issues with packet loss or acceleration so the measurement will not always line up with the calculation.
Siege and CSGO don't have any issues with this though. The issues here is the FOV as you assume. The best practice is to always use the same FOV, this is however not possible in all games.
I just removed it, changes in the config file are reverted when you launch the game so it doesn't work to set the sensitivity there unless we find a workaround.