This message is for wejones.
I found a link of yours in another post. You used small mirrors taped to your FC90 to find the focal point. I have a 90cm dish fitted with a multi-lnbf bracket. I tried your experiment and found that the point at which the reflections from the mirrors converged was an inch closer to the dish than the lnbf holder was set to.
So I adjusted the bracket to put the face of the feedhorn on the focal point to see if I could get a better signal (i.e. better performance from my dish). It turns out that instead of an inch, half an inch gave the best result. I used
G25 for my test.
I wonder if the focal point has to be at the face of the lnbf or at the "cross-hairs" near the back of it. I can post some shots if there is interest.