The FOV slider has a degrees symbol next to the value which would indicate it's measured in degrees, but the actual value it displays is a percentage.
The attachment FOV.png shows the internal FOV value compared to the value displayed in the game's options screen. Internally, the FOV is stored as degrees as a value between 30 and 110; in the options screen it's displayed as a percentage between 0 and 100, despite the degrees symbol next to the value.
Either the value displayed in the options screen should be in degrees, or the degrees symbol should be removed.
In 0.17.0.1 the FOV slider has been fixed. (The min is 30 degrees, the max is 110 degress, and the default is 70 degrees.)