amateur horn antenna
I agree with the answer of Kit-the-great.
I add some considerations.
If the feed is placed on primary focus, i.e the illumintion angle is "large" (F/D= 0.3...0.4), so the phase center is pratically placed on the aperture, and the usually accepdet tollerance is +/- 0.5 Lambda.
If the feed is placed on Cassegrain (or Gregorian) focus, you must know the position of phase centre, but fortunally the tollerance is mutliplied by magnification factor (tipically 10 for F/D=0.32) so the tollerance became +/-5Lambda.
As rule of thumb, for a 25 dB gain horn, the phase center is L/3 deep (where L is the distance Mouth-Thoat).