Your math is a bit off, the formula is like this:
atan(tan(x/2)*y)
Where
x = FOV in radians
y = matching percentage (75% = 0.75)
So in this case it's: atan(tan(100.8degrees/2)*0.75) = 42.2 (rounded).
As an example, CSGO's 90 FOV is expanded to 106.26 in 16:9, so you can calculate back to the 45 degrees 75% equals by doing so: atan(tan(106.26degrees/2)*0.75) = 45.
Hope that clears it up a bit.