How to derive camera FOV from the ratio of a projector's optics

hi, I was tring to set up cameraCOMP pars to be as the ‘view’ of the video projector,
to reproject it out and fit the real world, beause the measures and positions of virtual and real world matches. so I found a formula, changed a bit, and out with this, but i’m not so math.

ratio = 0.8
op(‘camera1’).par.fov = math.degrees(2 * math.atan((1 / 2) / ratio))

result 64.01077

Looks correct, is it?
I hope could be usefull for others, if right.
thanks