I was wondering about this myself since I know how to find the f/D ratio and focal length for a BUD but wasn't sure about a small offset dish.
My first FTA dish was a surplus 30" Alpha Star model from the failed DBS service in Canada. It had a Tee-Comm LNBF that worked so poorly all the signal strength readings were below 10% on
Telstar 5 and I couldn't get much of anything. I though the LNB must be bad so I got an Eagle Aspen model to replace it but that didn't work any better. They are little round cylinder shaped units with a round feed horn built-in on the front. I was looking carefully at it and it seems to me the horn is made for a bigger dish with a long focal length, like a 10' model. On the Alpha Star dish it looks like the feed will only see about 1/4 of the surface, which would explain the poor performance. I tried the LNBF from a Primestar dish with a wide angle feed and it worked great! The Hughes model from a DirecPC dish works good too. All I can conclude is the Alpha Star people, and Tee-Comm (who made the equipment) must be idiots, which would explain why Alpha Star failed.