I am returning to the same spot where I took nice pictures last Sunday but this time will use an APX 100 film instead of HP5. Usually I develop it in Studional (Rodinal special) 1+15 for 4 minutes in a JOBO. Since I want to print as large as possible, say 60x80cm, I need the finest grain possible. What is the rule of thumb when diluting a film double? In this case 1+30. Twice the normal time was not enough (8 minutes) the first time! Before testing and retesting again & again, I would like to get your thoughts regarding the same developper of course. Thank you!