before Feko, choose hyperboliod by simple geometric analitic concepts.
The hyperboloid has 2 foci. One of them is coincident with the paraboloid, the 2nd may be choosed on a convenient Cassegrain region.
Then by a simple ray tracing, find diameter of sub reflector.
Remeber: choose a convenient position of horn, as horn is far from subref as the horn has smaller beam so as the horn is larger.
Someone thinks the best subreflector angle seen by horn is around +/-15...20 deg.