Thanks Sander, I think we are on same track!
Good you added the problemes with air bubbles, forgot that!
And yes, I also think I normally use somewhat more solution than just above the film, I think I end up at 550, perhaps even 600 ml,
Your formulation: "2. make sure the tank is big enough to allow all developer to clear the film when the tank is fully inverted," points to why filling up the tank with rolls of film will give you problems, the "up going" airbubbles that manufactures talk about when inverting the tank is just not enough!
You also confirm my hesitaion about two rolls in 1 liter Paterson tank, even though it has a big mouth, but probably will not fulfill your point 2 which is most important.
I can also add that the rotating drum Jobo system (like my CPE-2) gives overdeveloped edges on 120-film. I try to avoid that by regularly inverting the drum during the development process, and it seems to work well for my C-41 developing process.