guid for design horn antenna

i simulated a horn antenna in feko at x band ferequency.
i know the feed should be placed at Lg/4 from end of rectangular waveguide and the prob should be L0/4 length.
but i dont know what should be the whole length of the rectangular waveguide for achieving minimum VSWR.
who can guide me PLZ?

hi
i think no one can said how we can achieve the lowest VSWR
but if u mean that u need VSWR less than 2 the length of waveguide should be between 24-26mm