I always measure sensitivity and FOV with scripts.
Aims that have blacked out the edges can be measured in two ways.
Either by figuring out how the game scales FOV and sensitivity through other tests. So for instance if the game scales sensitivity to 100% monitor match, I know that double the 360 distance equals half the horizontal FOV.
Or by measuring the distance visible in the scope. If only 40% (between the crosshair and edge) of the screen is visible, I can measure how far moving this 40% is, compare this to the 360 distance and calculate how many degrees I moved to reach 40% monitor distance. From there some more math to find the FOV.
The FOV in the calculator is for the entire screen regardless of what is blacked out.