prodeline dish1.8M

If it is 1184 antenna, with 22.3 degrees of offset, you should fix the feedhorn from the reflector at the following parameters: (unfortunately I don't have one nearby to give you the dimensions of the feed support tubes, and the manual wouldn't tell either).

Inclination of feedhorn: 41 degrees (upwards)
"A": (from the bottom of feenhorn to the bottom of reflector): 97cm
"B": (from top of the feedhorm to bottom of reflector: 112.1cm
"C": (from bottom of feedhorn to top of reflector): 181.9cm
"D": (from top of feedhorn to top of reflector): 172.1cm

Here are another set of A B C D measurements for an old Prodelin 1.8m antenna.

A=40.114 inches
B=44.107 inches
C=72.012 inches
D=69.310 inches

These relate to a Ku band transmit/receive feed.
Obviously you get different measurements for a large diameter C band feed or a tiny Ku band feed as used for receive only.

I have found that averaging, like so (A+B)/2 and (C+D)/2, gives two useful distances which define a single point at the centre of the feed window.   I suspect that the actual focal point (phase centre) lies a short distance down inside the horn, but this distance will vary from one feed design to another.  

Best regards, Eric.