Dish focus for a BSB Mini-dish?

Gareth G4XAT

I'm re-purposing a BSB Minidish for a new life on 122GHz where I will initially be using a Chaparral (Scalar) feed whose tech drawings indicate a 60 degree front lobe. I have an original Black BSB LNB so I can refer to where and how it is mounted. It has a plastic lens inserted into the LNB barrell, outer end is a cone, inner part turns into a blade arrangement. Something to do with the original circular polarisation perhaps?
But where might the actual dish focus be....before I add lots of aluminium tape to the dish and put it out in the sun and try that way....