Why dont you take a universal LNB and make a test? f/d around 0.6 usually
Adjust up and down until you get maximum signal strength.
I guess it would give better results than a feedhorn with f/d 0.4, gives you an indication in the right direction.
It is a little confusing but as I have understood it a feedhorn has a f/d value where it is peaking but you can also use it around that value.
Then the focalpoint can be at different spots inside the feedhorn, within a range for example 0.5-0.7 , this is at least my conclusion, please correct me if I am wrong.
If you have a dish with f/d 0.55 and use a horn with f/d 0.6 you must move the horn a little bit out compared to using a "perfect" feedhorn.
Go to satellite-calculations.com and calculate the opening angle for different f/d values.