final design of cassegrain antenna, need your opinion!

hi
i decide to designing a simple parabolic antenna with the geometric theory.
my limit is the main reflector size: 300mm.
the frequency is 100GHz.

i start from the F#. i know the range is 0.3-1, but i dont know how mach is impotent. i chose F#=0.5
so if my F# is 0.5 the f(main reflector) is 150mm.
now lets go to the hyperbola:
i think the limit hare its too things: one it is the feed(corrugated horn antenna witch illuminated 23(deg)) so this limit the f2(hyperbola) to tan(11.5)=(D/2)/f2).
if i chose the size of the sub reflector is 20% from the main so is: D~50mm
so the f2(hyperbola) is 150mm !
now i can decide the "e" parameter of the hyperbola (1.1-1.6). and from "e" i can get the -f1(hyperbola). i chose e=1.5
the distance between the mirrors is: f(main)-f1(hyperbola).
i put it on zemax and it looks ok !

i like to hare another opinion?
thanks a loot.