Fov and tunneling calcuation formula

[Hoody2shoes]

I cannot find the formula for calculating tunneling for the life of me. I know I've seen it before-can someone post it up please?

Edit***
Is it
Max fov/min=x
Erector/x=y
Y*base mag=min mag where tunneling begins?

 

[beetroot]

Max fov/min=x
Erector/x=y
Y*base mag=min mag where tunneling begins?

That looks about right. Although it's easier to just:

MaxMag x MaxMag FOV / MinMag Fov = Magnication that you get the lowest FOV (so where the tunneling ends).

 

[Hoody2shoes]

Thanks that's wayyyy easier