setting feed horn - again

[oldsatfun]

I have to adjust the feedhorn and the scalar ring of a Zinwell lnbf.
Has the focal point to be 1/4" inside the mouth of the feedhorn, or this is correct only for a Chaparral feedhorn ?
In other words, every lnbf/feedhorn is different or 1/4 inch inside the mouth of the feedhorn is always correct ?

My dish is a Fortec Star FS6D, modified to perfectly fit a Jaeger 1224 EL (dual axis HH).

 

[mkm4]

The focal point for different feedhorns should be similar, but remember they are adjustable for a reason. Your calculations are only a close estimate. It's still best to fine tune it.