how to choose the radius for cassegrain antenna

hii
i decide to go back and designing a simple parabolic antenna with the geometric theory. after that i check the Gaussian theory.
so i dont know ware to start:
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 importent.
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=60mm
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).
the distance between the mirrors is: f(main)-f1(hyperbola).
i put it on zemax and it locks ok !

so i dont know if all that trow, i like to hare another opinion?
thanks a loot.

I can see that you made the calculations; they look good; if your software confirmed the results, I think you ran a good trial.

In other messages you have mentioned you already have a real antenna at hand. I would suggest you measure its dimensions and use to run your calculation program again. If your software confirms the real dimensions are also good, then you know how to design a good Cassegrain!

hii
i can not measure the antenna that i have because the sub reflector is hidden behind some dielectric.
i dont understand what so impotent in the F#? and i see that i have 3 F#, main mirror sub reflector and feed horn,
whats the steps for good design?