"trackIR 6dof works just fine "
thats the key it works wrong in every game
as soon as you change distance to the screen the fov GOT to be modified accordingly, im sure the greeks would have done it right since the start
couldnt be easier and less resource consuming than a simple trig calculation
edit:
check the effect i explain here:
the crosshair has a constant angualr size whatever the distance of the camera: